Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

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CHAPTER

3.2

Mass Analysis and Partial

Pressure Measurements

Laszlo V.

Lieszkovszky

GE Lighting

3.2.1

OVERVIEW AND APPLICATIONS

There are numbers of cases where more information is required than a total pres-

sure gauge can provide. Often the real question is the composition, the change in

composition of the sample or the partial pressures of some constituents.

When the sample pressure is about atmospheric, one can choose from several

available techniques. Mass analysis is only one of them. At low pressures, or

when the sample size is small, however, partial pressure analyzers (PPAs) face

little competition. This section provides a general description of PPAs, identifies

some areas of application and defines some terms used for characterizing the in-

strument. Following sections describe the main parts of a PPA: the inlet system,

the ion source, the analyzer, and the detector. The inlet system converts the sample

pressure to a vacuum level acceptable for ionization and directs the sample to-

ward the ion source. The ion source creates ions from neutral species so that they

can be separated by electric or magnetic fields. The analyzer separates the ions

according to mass-to-charge ratio (M/Q; also called "mass number"). In the case

of spatial separation ions created at the same time are made to travel on various

trajectories according to mass-to-charge ratio. Ions with trajectories leading to

the detector provide an ion signal, others are lost. Separation in time is another

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290

3.2.1 Overview and Applications 291

method: ions created at the same time are traveling at different speeds according

to mass-to-charge ratio, reaching the detector at different times. Finally, the de-

tector converts the ion signal into easily measurable electric current. Advantages

and limitations of the choices are given and some basic principles discussed.

Mass analysis—like the most sensitive total pressure gauge, the ion gauge —

uses ions (in the vast majority of cases, positive ions) formed from the sample

atoms or molecules to provide a signal. Whereas the ion gauge measures all ions

without distinguishing them from each other, the analyzer section in a mass spec-

trometer will separate the species based on their mass-to-charge ratio (M/Q); e.g.,

a double-charged ion of the most common argon isotope "^^ Ar^"^ has a nominal

mass number M/Q = 20. The mass-to-charge ratio as the primary coordinate of

the measurement represents an important challenge of mass spectrometry: a sig-

nal at a given mass-to-charge ratio may not uniquely identify a species. (M/Q =

20 can be 20Ne+; M/Q = 28 can be C0+, N2^, Si

•",

or C2H6^; M/Q = 44 can be

C02^orN20+,etc.)

The issue can be even more complex: Ion species from one constituent appear

at various mass numbers not only because of isotopes, but also because various

species are produced by the ionization process

itself.

(Referred to as fragmenta-

tion:

e.g. CO2 may produce ions such as C

"^,

M/Q = 12; CO ^, M/Q = 28

0+,

M/Q = 16;

02^",

M/Q = 32; and C02'', M/Q = 44 "parent ion".)

Tables 1-5 contain normalized spectra of different substances (magnitude of

most abundant peak = 100). Atomic or molecular weight (M. Wt), relative sensi-

tivity of the most abundant peak compared to "^^ Ar^ (Rel. Sens.), and isotope ra-

tios are also listed. Spectra and relative sensitivity values are given for illustrative

purposes only—they depend on instrument design and settings. (Values given

measured at 70 eV electron energy. Am = 1 constant peak width mode of a

quadrupole mass spectrometer.)

Nonetheless, some experience, isotope tables, and standard spectra usually

help.

Despite (or in some cases because of) these intricacies, mass analyses have

been successfully employed in a number of applications ranging from air to vac-

uum.

3.2.1.1 Applications

Air pollutant measurement, when the composition of pollutants is unknown, may

present a problem for gas chromatography, that would require different columns,

detectors, etc. A freezeout-thermal analysis mass spectrometric technique is ca-

pable of identifying impurities such as various freons and hydrocarbons at sub-

ppm levels [1]. In searching for abnormal or possibly toxic pollutants, the use of

cluster analysis program and standard library spectra may be of

help

in their iden-

tification or at least classification [2]. The calibration for measuring unstable at-

292 Chapter 3.2: Mass Analysis and Partial Pressure Measurement

mospheric constituents such as ozone requires special procedures [3], and so do

the mass spectrometers flown in balloons to measure altitude profiles of standard

constituents [4].

Environmental characterization and monitoring of hazardous constituents in

the field typically involves a gas chromatograph-mass spectrometer combination

(GC-MS), where the GC is used for coarse separation, the MS for fine separation

and detection. Portable instruments are available for site characterization using

quadrupole analyzers or the quadrupole ion trap [5]. (See end of Section 3.2.4.1

about the quadrupole ion trap.)

Biochemical analysis includes fermentation, environment monitoring and con-

trol, enzyme kinetics for determining velocity constants and reaction mechanisms,

plant physiology-gas metabolism measurement, etc. Membrane inlet mass spec-

trometry has become a well-established technique capable of on-line monitoring

of gases and small-molecular-weight organic compounds dissolved in water. If

organic molecules are leaked into the vacuum chamber through a membrane, the

signal is the superposition of two transients, one reflecting the membrane response

and the other reflecting the adsorption/desorption processes on the vacuum walls.

Therefore the design of the inlet system is critical [6]. Multicomponent analysis

with multichannel sampling from the gas and the liquid phase is capable of mon-

itoring or controlling fermentation processes

[7,8,9].

In the research of plant phys-

iology a suitable sampling unit enables in vivo measurements without influencing

gas metabolism. Further example is the analysis of the effects of agricultural

chemicals on crop plants [10].

Isotope ratio analysis is a classical field of magnetic sector mass spectrometry

but quadrupole type is also used [11]. It is still constantly refined to measure iso-

topic effects in noble gases for study of natural or laboratory-induced nuclear

processes [12], or the study of isotopes of light elements such as hydrogen, car-

bon, oxygen, sulfur, and nitrogen in terrestrial or extraterrestrial geologic sub-

stances [13]. Quadrupole-type instruments have been tested for possible nuclear

safeguard applications requiring mobile uranium isotope measurement. With

proper settings, a resolving power of 233 and an abundance sensitivity of 3500

were achieved [14]. Clearly, isotope ratio analysis is an area where the above-

mentioned complications regarding spectrum interpretation may arise (e.g., oxy-

gen isotopes ^^O, ^^O, ^^O measured at M/Q = 32, 33, 34, 35, 36 results in a set

of linear equations —

the

molecules ^^O^^O and ^"^O^^O both contribute to the

peakatM/e = 34)[15].

Gas composition and purity analysis of bulk gas samples usually use a batch

inlet to determine gas composition or trace impurities. Gas composition analy-

sis usually requires careful calibration procedures to assure quantitative results

[16] primarily due to the fractionating versus nonfractionating gas flow in the in-

let or the vacuum system [18]. Contaminants down to about 10 parts-per-million

(ppm) range can be routinely measured. Analysis of electronic gases presents spe-

3.2.1 Overview and Applications 293

cial challenges due to the corrosive, toxic and reactive nature of the gases in-

volved. The sample-related problems are loss of minor constituents and addition

of contaminants as a result of adsorption/desorption processes, reactions with in-

strument components possibly leading to performance degradation, ion molecule

reactions and release of gases by electron, ion, or neutral molecule bombardment

of surfaces [19]. Techniques to minimize these effects include fast gas flow in the

sample-handling system, differential pumping of ionizer and mass filter to main-

tain high pressure in the ionizer while keeping the analyzer pressure low and beam

chopping with lock-in detection to be able to differentiate between sample signal

and background [20]. Below the 10 ppm level, special procedures are required,

such as the preconcentration techniques either using freezeout (He in N2, and H2

in Ar) [21] or chemical methods (measurement of trace impurities in oxygen by

chemically removing the oxygen using NaK alloy) [22] or in some case increase

of the high-pressure operating limit of the spectrometer (CO2 in N2) [23]. When

trying to achieve parts-per-billion level, a combined gas chromatograph-mass

spectrometer system has been used successfully for trace gas detection in high-

purity gases used for semiconductor fabrication (H2, N2, CO, Ar, SiH4, N2O,

etc.) [24]. The most sensitive method available uses the preferential ionization of

impurities by charge transfer mechanism in the high-pressure chamber of the at-

mospheric pressure ionization mass spectrometer (APIMS) allowing contaminant

measurement of ultrapure gases at the parts-per-trillion levels—representing the

state of the art in trace gas analysis today [25].

Leak detection is a built-in feature of many contemporary partial pressure ana-

lyzer systems. Finding leaks or verification of vacuum tightness is an omnipresent

problem of vacuum work and there are cases where traditional helium leak detec-

tors prove to be an inferior solution to mass analyzers. In large and complex vac-

uum systems such as the Gas Centrifuge Enrichment Plant/Ohio consisting of

1000 miles of piping and 10^ L vacuum volume, the difficulty of hooding and

permeation of helium through O-rings makes traditional leak testing a very ineffi-

cient method. Instead, a special air leak test cart has been built that can be con-

nected to large number of ports of the system and enables isolating leaky sections

by manipulating valves in the area [26]. Leak testing of hermetically sealed prod-

ucts can also be realized for quality checking at a rate of 6000 pieces/day (leak

testing of capsules filled with freon) [27].

Medical applications are exemplified by diagnosing certain diseases by observ-

ing the response of the respiratory system to gases of various solubilities in the

bloodstream or monitoring the concentration of anesthetic gases (such as halo-

thane, penthrane, nitrous oxide) for the safety of the patients and the operating

staff

[28,29].

Gas probes for blood gas monitoring can be of

the

membrane

type

—

an in vivo catheter or skin transducer measuring the gas diffusing through the

skin. Requirements are fast response time to sample changes, stability in perfor-

mance [30,31], and thorough understanding of the inlet system behavior [32].

294 Chapter 3.2: Mass Analysis and Partial Pressure Measurement

Metallurgical applications have to compete with infrared absorption instru-

ments for the determination of carbon monoxide and carbon dioxide. The advan-

tages offered by the mass analysis technique for effluent gas analysis, vacuum

metallurgical or steelmaking process control are the speed and complete and con-

tinuous analysis of multicomponent mixtures by one instrument (including direct

determination of nitrogen)

[33].

The difficulty of the interpretation of overlapping

peaks of two important gases, nitrogen and carbon monoxide {M/Q = 28) ap-

pears again, but the dependence of the covariant matrix on electrical settings of

the spectrometer can help to resolve specific problems and achieve maximum ac-

curacy [34]. Mass analysis has been used to analyze the finished product as well,

for example, gas contents of steel or other metal products are measured by melt-

ing the samples and simultaneously determining carbon monoxide, nitrogen, and

hydrogen in the few-parts-per-million range [35].

Outgassing and permeation rate measurements routinely include a mass ana-

lyzer to determine the principal gas species contributing to the measured out-

gassing rate of various metals and ceramics [36]. In the measurement of gas

emissions from plastics (Plexi, polystyrol. Teflon) [37] and elastomers (Viton,

Perbunan, Silastomer, and other organics sometimes used in vacuum work such

as Araldite, Nylon, polyethylene, PTFE) [38], knowledge of the mass numbers of

the outgassing species is at least as important as the rate of outgassing — although

with the exception of polyethylene and PTFE the main component in most cases

is water.

Plasma-related applications of mass analysis range from identification of ion

species for the study of gaseous plasmas (NeAr and NeKr) [39], sampling of glow

discharges, investigation of the gaseous electronics aspects of sealed CO2 laser

operation [40] to partial pressure or flux analysis of fusion devices (Tokamak

Fusion Test Reactor/New Jersey) [41], (Axially Symmetric Divertor Experiment/

Germany) [42] (Joint European Torus) [43]. Mass spectrometers are used here as

a diagnostic tool for optimizing and process-monitoring glow discharge wall-

conditioning procedures in the various cleaning scenarios such as pulsed dis-

charges or DC-glow discharges. (Fusion experiments themselves apply strong

magnetic fields, making it impossible to use standard analyzers close to the plasma

vessel.) The two basic categories of such investigations are (1) partial pressure

analysis, sampling of volatile reaction products representative of the integrated

effects of plasma-wall interactions and (2) plasma flux analysis, sampling neu-

tral plasma particles streaming directly into the spectrometer [44]. Although the

latter requires more sophisticated vacuum connections and coupling between the

plasma and the spectrometer, it allows energy analysis of the plasma species.

Since ion sources are operated usually near the high-pressure limit (10""^ torr,

^10"^

Pa), hydrogen conditioning can be employed to reduce residual gas levels

[45].

Long-term changes in sensitivity were reported to be reduced by the use of

tungsten filament (as opposed to rhenium) and operation at a constant tempera-

3.2.1 Overview and Applications 295

ture [46]. Peak overlapping in hydrogen, deuterium, tritium plasmas (H2, D2, T2,

HD,

HT, DT, "^He, ^He all appearing in the MIQ range 1-6) can be resolved by

using the dependence of appearance of species on instrument settings (electron

energy setting offering separation of helium and hydrogen—and in case of qua-

drupoles — field axis potential setting offering separation of molecular ions from

fragment ions) [47].

Process control may be the single largest area of application. Quality of the

seven main components of natural gas have been monitored to control mixing

stations using mass analysis much faster or more thoroughly than alternative

techniques (gas chromatography, infrared). The general features that make mass

analyzers attractive as a process technique is the speed of analysis, versatility—

capable of being adapted to a wide range of analyses with little development work

involved (as opposed to gas chromatography), ruggedness (of specially built in-

struments) and reliability. Regarding cost, mass spectrometric technique is gener-

ally more expensive than existing specific instrumentation and there are some

limitations in its specificity (e.g., ability to distinguish between isomers) [48].

There is one area, however, where its widespread use is unquestionable —

semiconductor processing and thin-film deposition in general. The applications

of mass analysis include monitoring or closed-loop control of sputtering and chem-

ical vapor deposition (CVD) processes. End point detection in plasma-etching

processes is one of the most important features. In sputter gas atmospheres, the

typical pressure approaches 10"^ torr (~1 Pa) commonly requiring a pressure re-

duction step (orifice) and choking of the vacuum pump to minimize effects of

preferential pumping. In situ calibration of the sensitivity factor of

the

mass spec-

trometer enables identification of the contaminants that may have considerable

effect on semiconductor properties [49]. Oxidizing constituents of the sputter at-

mosphere such as water and oxygen generally must be monitored at the ppm level.

The benefit of connecting the sputter gas atmosphere without pressure reduction

step is the avoidance of large errors in the measurement of background gases

[50].

Recent approaches expose the ion formation region directly to the high pro-

cess pressure while the filament and the mass analyzer kept at high vacuum are

connected through small orifices to the ion formation region [51]. Removing hot

filaments from the ionizing region reduces process gas-filament interaction, and

reduces the chance of instrument performance degradation (e.g., metal deposit on

insulators when using WF6). Also, stability is improved, since pumping varia-

tions have very little effect on sample pressure, while variations in sample pres-

sure have little effect on electron production or ion separation efficacy. The con-

tamination resistance of the analyzer section and its abundance sensitivity can be

further improved by the use of a triple-filter arrangement where the "pre" and

"post" filters are driven by RF (radio frequency voltage) only [52]. Real-time

monitoring of reaction products provides an opportunity to dynamically optimize

process parameters. The basis for end point detection is the rate of evolution of

296 Chapter 3.2: Mass Analysis and Partial Pressure Measurement

reaction products from the reaction between the substrate and etch gas mixture

(such as SiF4 evolution while etching Si02 with perfluorocarbons, or as the for-

mation of CO2 that provides a signal for end point detection of photoresist strip

with freon-free oxygen plasma) [53].

Mass spectrometers of the quadrupole type are also used as rate sensor in the

feedback loop in UHV deposition processes (for example, electron gun evapora-

tion in molecular beam epitaxy (MBE) systems). Noted advantages are the large

sensitivity, large bandwidth, and multiplexing capability (monitoring several

sources simultaneously by switching between different mass numbers), large dy-

namic range for rate measurement, and low maintenance needs. Limitations are

the possible instability of this complex instrument, the fact that a separate flange

is required where the instrument has to be inserted relatively deep in the chamber.

In addition, the viewing angle may not be wide enough, the point of deposition

may be farther away from the point of measurement, and the stabilization time of

the mass spectrometer at the new mass number may be long in multiplexer mode

[54,55].

For identification of vapor species present and particle density measure-

ment in an MBE apparatus, a time-of-flight technique has been developed [56].

Mass spectrometric contamination control of process gases used in electronic

device fabrication requires the solution of several issues related to reactive and

corrosive nature of these gases. The memory effects, surface interactions, and re-

actions in the ion source or analyzer region and pumping system must be mini-

mized along with the possible ion-molecule reactions. A proposed system con-

sists of a molecular beam inlet system with a mechanical chopper and lock-in

detection with a combination of turbomolecular and two-stage He cryopump —

the 14 degree K surface enclosing the ion source is needed to assure low back-

ground [57].

A technique called electron impact emission spectroscopy must be mentioned

here.

It allows selective monitoring of partial pressures of species by detecting

their optical emission lines as a result of excitation by electron bombardment.

This approach is used in some partial pressure controllers as well as in certain

vacuum deposition monitors. The simple calibration procedure, high stability, and

its detection system's insensitivity to electrical noise or charged particles (such as

in electron beam evaporation sources) offer advantages over mass spectrometric

approaches [58,59,60].

Sealed gas sample analysis by breaking, piercing, or melting the sample are

common in various industries such as microelectronics, glass, or lighting prod-

ucts industry as a quality check or research/development tool. In semiconductor

devices moisture is generally the biggest concern, which may provide surface lay-

ers,

unwanted electrical conductivity, and undesirable intermetallic growth, even-

tually leading to corrosion and failures. Even though nondestructive methods are

available (surface conductivity sensor, capacitance ratio test), and destructive op-

tical methods (such as infrared spectroscopy or more recently tunable diode laser

3.2.1 Overview and Applications 297

absoq)tion and intracavity laser spectroscopy) exist [61J, mass spectrometry with

electron impact or chemical ionization is preferred as the tool for package tech-

nology development or failure analysis [62]. The reasons are primarily its sensi-

tivity and ability to provide information on species beside moisture that may be

present due to poorly cured resins, use of cleaning solvents, desiccants, etc. Some

laboratories routinely monitor 32 mass numbers [63]. The principal problem con-

cerning the accuracy of the measurement arises from wall effects — adsorption/

desorption of moisture and other nonvolatile components. Of primary importance,

therefore, is proper "burst" calibration, which imitates the puncturing of the pack-

age and the subsequent release of its gaseous content either by moisture standard

packages [64] or using controlled moisture delivery apparatus with properly oper-

ated valve arrangements [65,66]. Another approach tries to break away from the

wall effect problem by converting the moisture chemically and quantitatively to

generate simple nonadsorbing gas products. Reagents used are sodium or calcium

hydride giving hydrogen, magnesium nitride or sodium amide giving ammonia,

and calcium carbide giving acetylene [67].

Measurement of gases entrapped in glass is of interest for the glass industry.

The analytical procedure is further complicated by the effect of mechanical shock

when breaking open the glass (desoiption) and the surface effects related to the

freshly broken glass [68]. Although little adsoiption of either nitrogen or oxygen

was noted by freshly broken glass surface, the adsorption of

SO2,

CO2, and mois-

ture can be a significant source of error. The observed sudden drop of CO2 sig-

nal is accompanied by the appearance of

CH4

—

conversion reaction has been

proposed as an explanation [69]. The two basic schemes used to analyze such

samples are the so-called "static" when the vacuum pumps are valved off and the

"dynamic" when the vacuum pumps are on [70].

Analysis of electric light sources raises similar concerns. To extend the sensi-

tivity into the sub-ppm range, a cryogenic preconcentration technique can be used

for incandescent lamps [71]. In this industry moisture measurement is also of

great interest, being the cause of blackening in incandescent lamps, possibly short

life in halogen lamps, and a cause of starting problems in discharge sources. Sev-

eral sampling techniques have been described to determine the moisture content

of the hygroscopic mixed iodide ampoules used in dosing of metal-halide dis-

charge lamps [72]. In these products as well as in sealed electrical contacts (Reed

relays) the sealing process itself will change the contaminants through various re-

actions at the high temperature but the measured impurities will provide a basis

for troubleshooting (e.g., CO, CO2 indicative of

O2,

etc.) [73,74]. Measurements

of gas fill of laser fusion target pellets is a case of extremely small gas quantity

determination (<10~'^ L) where a closed volume mass spectrometric technique

has been used

[75].

Commercial and custom-made systems are available for analy-

sis of trapped gas samples (IC packages, lamps, inclusions in glass) featuring de-

tection limits in the low-ppm range, and sample volumes down to 10"^ liter [76].

298 Chapter 3.2: Mass Analysis and Partial Pressure Measurement

In case of glass, an alternative technique to mechanical breakers and piercing

devices is to melt the sample in a furnace unit. In case of

metals,

this is practically

the only way to measure the amount of impurity gases trapped. Measuring the de-

composition products recorded with heating time provide information on glass

types suitable for certain applications [77] or aid in the development of metals

[78,33].

Vacuum applications are broadly defined; the largest portion being what is of-

ten referred to as residual gas analysis. Indeed, mass analyzers of limited mass

range (1 to a few hundred amu) are also called residual gas analyzers (RGAs),

implying the typical application. Such instruments can be found on a majority of

vacuum systems, since they significantly enhance the ability to improve basic

pumpdown conditions or allow understanding of phenomena in vacuum. As soon

10""*

torr (~10~^ Pa) has been reached, an RGA can indicate the possible ex-

istence of leaks, enables monitoring the composition and limiting factors of the

ultimate vacuum reached, and optimization or automation of

processes.

He-tuned

RGAs may prove to be a simpler and more cost-effective way to check vacuum

tightness than dedicated leak detectors [79]. Mass analyzers are indispensable

tools in analyzing the behavior of vacuum chambers and small systems [80,81]

or examining pump performance and effects of operational conditions of turbo-

molecular pumps [82]. Unfortunately, since the analyzer is part of the system its

presence influences the residual gases that would be present without it and this has

to be taken into account [83]. These applications commonly use low-resolution

analyzers and interpretations must be based on assumptions especially in case of

overlapping peaks. Only high-resolution residual gas analysis can reveal, for ex-

ample, that the common M/Q = 28 peak on a typical liquid-nitrogen-trapped oil

diffusion system is in reality a multiplet (CO, N2, C2H4) and their ratio is

markedly different on oil versus mercury diffusion pumped systems [84]. Large

vacuum systems such as the Synchrotron Radiation Source [85] or space simula-

tion facilities [86] also rely on RGAs for analysis or monitoring of facility condi-

tions and identification of contamination products.

Various other applications are reported in basic or applied research such as

the study of chemical reactions by means of coupling a microreactor to the mass

spectrometer [87] or the study of reaction kinetics in cases where practical rea-

sons prohibit the use of absorption spectroscopy (e.g., the case of high-melting-

point solids that are insoluble in most solvents such as tungsten halides) [88].

Coupling the mass spectrometer to a thermoanalyzer will provide information

on the gases evolving during the heating process—coupling combinations have

been proposed for temperatures up to 1400°C in vacuum and in gas atmospheres

as well as ways to reduce ion source contamination using cryosurfaces [89].

Pyrolysis study of materials — among others, study of degradation of PTFE

(polytetrafluoro-ethylene) [90]—is another example of controlled heating/mass

analyzer combination. Investigation of flames, understanding the processes in

3.2.1 Overview and Applications 299

combustion engines often uses modulated molecular beam technique to allow the

detection of highly reactive particles, radicals or condensable species [91]. Study

of neutral and charged particles from a transient event where the exact timing and

position are not known requires a fast-responding technique with easy triggering.

Time-of-flight mass spectrometry has been used for the electrical breakdown of

insulating surfaces (thin polymer films) [92]. Investigation of the lifetime, laser

output, and gas composition changes of gas lasers has been using various tech-

niques such as spectroscopic and chromatographic analysis, but the mass spectro-

metric technique has the advantage of analyzing many components simultane-

ously in real time with the disadvantage of depletion of gas from the laser tube.

To minimize gas losses, a special inlet system such as an extremely-low-conduc-

tance on/off valve with zero dead volume may be necessary [93].

Mass analysis using MS-MS (mass spectrometer-mass spectrometer), triple

quadrupole, or ion traps are widely used in research and routine organic analysis

(such as the ion trap as a detector of chromatographs) but these are beyond the

scope of partial pressure measurement related to vacuum technology.

3.2.1.2

System Performance

The performance of the mass analyzer system is evaluated based on a number of

characteristics. Which is important, depends largely on the application. For quan-

titative measurement, a proper calibration procedure is necessary. Recommended

practice and methods of calibration as well as definition of the terminology can

be found in a recent update of the AVS Standard 2.3-1972 (1991) [94]. Some im-

portant considerations are listed here:

Mass range: The range between the minimum and maximum mass numbers,

over which an instrument operates.

Sensitivity: The ratio of change in ion current, or peak height due to the corre-

sponding change in partial pressure of

the

particular species (A/torr or

A/Pa).

The

main factors influencing sensitivity are the emission current, the resolution setting

of the analyzer, and the gain of the multiplier if used (see Sections 3.2.3-3.2.5).

Abundance sensitivity: A term frequently used in isotope measurements; refer-

ring to the suitability of the instrument to measure rare isotopes adjacent to abun-

dant ones. It is the ratio of the peak height of the abundant isotope at M/Q and the

smallest detectable ion current of the rare isotope at the subsequent mass number,

M/Q + 1.

Values in the range of 10'^-10^ are typical. This figure of merit is related to the

resolving power and the sensitivity of the instrument. Tailing contribution is an-

other term to describe the residual signal from a major peak at the preceding and

following mass number from the center of the major peak.