Instrument-Independent MS/MS Database for XQQ Instruments: A Kinetics-Based Measurement Protocol.

A detailed kinetics-based measurement protocol is proposed for the development of a standardized MS/MS database for XQQ tandem mass spectrometers. The technical basis for the protocol is summarized. A CAD database format is proposed.

1. Introduction

Tandem mass spectrometry (MS/MS) is widely used for structure elucidation and for the analysis of multicomponent mixtures [1]. The analysis makes use of the collisionally-activated dissociation (CAD) of “parent” ions. A parent ion may be a molecular radical cation, a protonated molecule, or a “progeny” fragment ion (daughter, granddaughter, etc. produced by the fragmentation of a larger precursor parent ion). The MS/MS technique consists of three steps: (i) a parent ion is mass selected by the first mass analyzer (designated MS-I); (ii) the selected parent ion collides with a target gas which is located within a reaction region between MS-I and the second mass analyzer (designated MS-II); and (iii) the undissociated parent ions and the progeny fragment ions which were produced within the reaction region are channeled into MS-II for mass analysis. MS/MS instruments thus produce a CAD spectrum of each initially-selected parent ion.

Reference [1] provides an excellent up-to-date review of the capabilities and advantages of the MS/MS technique. Among the advantages enumerated in [1] for MS/MS are:

the functional group specificity of neutral loss experiments (for which there is no analogue in GC/MS) provides the unique ability to rapidly screen unknown mixtures for compound classes (e.g., nitroaromatic cations can be inferred by their loss of NO).

the ability to perform very rapid analyses of targeted compounds in complex mixtures based on unique progeny ions (especially when “soft” ionization techniques are used) or on neutral loss experiments [e.g., the nitroaromatics screened above in (a)], thus avoiding the need to identify every component.

very low limits of detection (≤ppt; better than a single stage of mass spectrometry) can be attained with enhanced signal/noise ratios because of the elimination of “chemical noise” [1].

unstable or extremely reactive species, which cannot be studied directly by optical spectroscopies, NMR, or GC/MS, have been studied directly by MS/MS (e.g., radical trapping of polyatomic free radicals; neutralization-reion-ization mass spectrometry [1]; etc.).

In principle, standard CAD spectra of a variety of ions (fragment ions, molecular ions, protonated molecules, etc.) could be generated and collected as reference libraries, to be used for comparison against unknown spectra in a manner analogous to the use of reference libraries in the data handling systems of ordinary electron impact mass spectrometers. Further, it should be possible to infer the identity of an unknown molecule by identifying the ionic substructures of fragment ions generated in its CAD spectrum. However, to date reference libraries of CAD spectra have not been collected because of a lack of standardization of operating conditions of MS/MS instruments [1].

This paper addresses the standardization of one of the major classes of commercially-available analytical MS/MS instruments, the so-called “XQQ” tandem mass spectrometers (there are currently more than 400 XQQ MS/MS instruments worldwide, representing a capital investment in excess of $170 million). XQQ instruments have three components: XX Q2 Q3. X1 designates the first mass analyzer, which can be a quadrupole mass filter (represented by a Q), a reversed-geometry magnetic/electrostatic sector instrument (represented by BE), etc. Q2 designates a quadrupole mass filter operated with only radiofrequency (rf) potentials [no direct current (dc) potentials are used]. Q2 contains the target gas, and provides efficient ion containment of parent ions and of progeny fragment ions. Q3 stands for the second mass analyzer, a quadrupole mass filter.

Because XQQ instruments (QQQ, BEQQ hybrid, etc.) are complex ion-optical devices [2]–[9], the observed spectra can be extremely dependent on the experimental parameters used during the analysis. That is, one observes instrument-dependent CAD spectra. This was clearly demonstrated in a 1983 international round robin [10] wherein very different CAD spectra were observed for the same molecule. That is, the relative intensities measured in different QQQ instruments for any given pair of progeny ions differed by factors ranging into the hundreds, even though the same nominal operating conditions were supposedly used in each of the QQQ instruments. Therefore, until now it has not been possible to use a CAD spectrum of a given species in one XQQ instrument to identify and quantitate that same species in a different XQQ instrument.

To develop an instrument-independent MS/MS database (or library) for XQQ instruments, instrument users must be able to tune their instruments to obtain undistorted, dynamically-correct (i.e., instrument-independent) representations of any reactions studied within such instruments [9]. That is, the measurements must be substantially free from kinetic interferences (viz., every effort must be made to ensure a well-defined gas target and to prevent back reactions, impurity reactions, scattering losses, fringing fields, mass discrimination, etc.). The branching-ratio measurements must be precise and accurate (±10%).

Recent work from this laboratory has explored the prerequisite conditions for obtaining instrument-independent dynamically-correct branching ratios for the CAD of ions within XQQ instruments [9]. In this paper a kinetics-based measurement protocol is described for the determination of standard CAD spectra within XQQ instruments. The technical basis for the protocol is summarized. Table 1 includes a typical CAD spectrum measured using this protocol.

Table 1

Proposed CBRIS1 database format2,3

Parent Ion:Source Compound:Ionization Mode:

C2H3O+(m/z43)4,52,3-butanedione (99.9%)70 eV electrons

Notes: 9, 99, 9996

CBRIS = C haracteristic B ranching R atios of I onic S ubstructures.

For this example we show the branching ratios (∝ − η) vs the center-of-mass collision energy (ECM) for the CAD of (C2H3O+) from the source compound biacetyl [16].

The numbers in the square brackets represent the maximum uncertainty in the cross section σ and in the branching ratios (∝ − η), expressed as a percentage of each σ and of each branching ratio {e.g., for biacetyl at ECM = 2.4 eV, the maximum uncertainty in y is ±([2%]/100)γ; i.e., γ=0.999±0.02}.

A reference citation would be provided for each CAD spectrum to identify the source of the data.

For the CAD of any given parent ion (e.g., C2H3O+), appropriate corrections must be made for contributions from the concurrent CAD of isobaric ions (e.g., C3H7+), regardless of their source. The isobaric ions may be co-produced in the ion source (i) from the source compound (e.g., ionization of CH3C(O)C3H7 will produce both CH3CO+ and C3H7+) and/or (ii) from a neutral impurity in a source compound (e.g., for a butanol impurity in a 2-butanone source compound, the butanol generates C3H7+, while the 2-butanone generates CH3CO+). Fortunately, the CAD spectra of CH3CO+ and C3H7+ are easily distinguishable [16]. That is, for the CAD of C3H7+, C2H3+ (m/z 27) is the major CAD fragment for ECM≃2−80 eV. By contrast, for the CAD of C2H3O+, C2H3+ is not produced at ECM = 2.4 eV, and is only a very minor fragment for ECM > 2.4 eV. Unfortunately, even minor impurities can contribute disproportionately to the CAD spectrum of a source compound because of differences in the CAD dynamics of isomeric and/or isobaric ions. Because of this problem, it is advisable that both the source compound and the target gas be of high purity (>99.95%). Otherwise, the impurities must be characterized so that appropriate corrections can be made for their contribution to the observed CAD spectrum.

The Notes field would be used to refer a user of the database to any information of special significance about the parent ion (e.g., structure of the ion, etc.). The notes enumerated in all the Notes fields of the CBRIS database would be collected together in a separate “Notes Appendix”. For the example given here, Notes 9, 99, and 999 would be found in the Notes Appendix, and might contain the following types of information.

Note 9: For a given ECM, the branching ratio for each fragment ion is substantially the same for all CH3CO—X source compounds (e.g., biacetyl, acetone, acetophenone, etc.) [16]. The reactant ion entering Q2 appears to be pure CH3CO+ in every case [16].

Note 99: The energy dependence (magnitude and direction) of the branching ratios is distinctly different for the isobars C2H3O+ and C3H7+ [16]. Hence, one can readily distinguish C2H3O+ from C3H7+.

Note 999: One can readily distinguish ethanol, oxirane, and cis-2,3-epoxybutane from each other, and from CH3CO—X source compounds on the basis of the energy dependence of the branching ratios for the CAD of C2H3O+ [16].

This protocol can also be used for the development of a standardized MS/MS database for XQQ instruments. One of the goals of this paper is to promote discussion about the database format best suited to the needs of the analytical mass spectrometry community. The spectrum of table 1 illustrates the database format proposed here. The precepts of the protocol are also applicable to other types of tandem mass spectrometers which have strong focusing properties (e.g., quadrupole-hexapole-quadrupole MS/MS), so long as the energy range used for the pertinent ion chemistry is the same as for XQQ instruments.

The kinetics notation and nomenclature developed in reference [9] are used throughout this paper.

2. General Background

As has been demonstrated [11]–[16], dynamically-correct branching ratios can be measured in XQQ instruments when the key MS/MS instrument parameters [9] are properly selected to account for reaction-induced mass discrimination [9] within Q2 and the intrinsic mass discrimination within the Q3 mass analyzer.

The systems studied in this laboratory include charge transfer and dissociative charge transfer reactions, and CAD.

Charge Transfer: [11], [12], [13]

Dissociative Charge Transfer: [14]

CAD: [15] [16]Note that for collision energies ELAB≃1 – 200 eV in the Laboratory (LAB) frame of reference, charge transfer reactions are experimentally equivalent to a “worst-case” CAD reaction system because they take place at large impact parameters with near-zero momentum transfer. That is, the product ions of a charge transfer reaction are formed essentially at rest (thermal energies) within Q2 [17]–[20].

The studies in our laboratory led to the development of a kinetics-based measurement protocol for the generation of standard XQQ spectra. Using the protocol, one can obtain, for the first time, accuracy and precision for CAD measurements within XQQ tandem mass spectrometers. A round robin exercise, the NIST-EPA International Round Robin [21], was organized to ascertain which commercially-available XQQ instruments are capable of generating dynamically-correct (i.e., instrument-independent) spectra. Appendix 1 contains the actual round-robin protocol which was disseminated to 22 laboratories worldwide (including the six XQQ instrument manufacturers). Analysis of the round robin data from six participants indicates that at least 50% of the QQQ instruments which have been sold and are currently in the field can provide such standard spectra [21].

Instruments which can generate dynamically-correct results with the round robin protocol of Appendix 1 can be used to develop a generic, standardized CAD spectral database (or library) based on Characteristic Branching Ratios of Ionic Substructures (CBRIS) (analogous to the use of group frequencies in infrared spectroscopy; see section 5.1). Hence, members of the analytical mass spectrometry community who have such instruments could use the kinetics-based measurement protocol detailed in this paper to generate and contribute instrument-independent CAD spectra of species studied during the course of their work. Contributed CAD spectra (to be sent to this author) would be included in the NIST-EPA Standardized CAD Database currently being developed in our laboratory. The latter would be disseminated by NIST to the analytical mass spectrometry community. To facilitate the critical evaluation of contributed spectra, a dynamically-correct CAD spectrum of a well-studied “model” compound (e.g., n-butylbenzene) should also be submitted.

3. Technical Basis for Protocol

Here we summarize the technical basis for the kinetics-based measurement protocol. For a more detailed treatment, the reader is referred to references [2]–[9].

3.1 Kinetics

With reference to the following general reaction sequence: equations (1)–(3) are applicable under pseudo-first order {[B]0>>[A+]0}, single-collision conditions for a reaction zone of length L wherein the number density of the target gas is [B] and the “target thickness” is [B]L. Here σ (= ∝ σ+βσ + …) is the total cross section for the A++B interaction, and the sum of the branching ratios ∝ +β + … is equal to 1.

Reactant Ion Decay: andProduct Ion Formation: etc.In the case of CAD, A+ corresponds to the parent ion; B corresponds to the target gas; C+, D+, etc. are the progeny fragment ions; and S, T, etc. are the neutral fragments complementary to C+, D+, etc.

Under dynamically-correct conditions, the rate of reactant ion decay equals the rate of product ion formation. That is ln Y = ln W∝ = ln W, etc. Then the product ion intensities C+, D+, etc. are related to the extent of consumption of the reactant ion {[A+]0 − [A+}by eqs(4),(5), etc. etc.

3.1.1 Dynamically-Correct Representation

If the key MS/MS parameters [9] of an XQQ instrument can be tuned so that data generated by the instrument conform to equations (1)–(5) [viz., so that ∝ + β + γ + … = 1.00], then the instrument can provide a dynamically-correct representation of any reaction studied within it. That is, by using eqs (4′). (5′), etc., the instrument would provide a measure of the branching ratios equivalent to those that, in principle, would be observed at the scattering center of a molecular beam machine. etc.

3.1.2 Kinetics Constraints

The selection of key MS/MS parameter settings is constrained by the kinetics prerequisite for ∝ + β + γ + … = 1 (see reference [9]). This prerequisite necessitates that:

each product ion be formed only by the primary reaction (no secondary sources; pseudo-first order, single-collision conditions)

all ions be detected with equal sensitivity (requires Conversion Gain corrections for each detector; see sec. 4)

there be no scattering losses because of unreac-tive collisions (must have high ion containment within Q2; approximately 100% collection efficiency for product ions and unreacted projectiles)

corrections be made for differences in ion containment (transmission) within the Q2Q3 structure.

3.2 Instrumental Parameters

To accomplish ∝ + β + γ + … = 1.00, the key MS/MS parameters must be properly selected to obviate discrimination against product ions because of: We shall use the terms “RIMD-free” and “IMD-free” to refer to branching ratios ∝, β, etc. which have been measured in concordance with the precepts detailed next for (a)–(c). That is, to be RIMD-free and IMD-free, the instrument parameters discussed below in sections 3.2.1–3.2.3 have to be tuned and returned iteratively until ∝ + β + γ + … ≃ 1.0±0.1.

Reaction-Induced Mass Discrimination (RIMD) within Q2 (refer to sec. 3.2.1),

Intrinsic Mass Discrimination (IMD) within Q3 (refer to sec. 3.2.2), or

the kinetic energy of product ions entering Q3 (refer to sec. 3.2.3).

3.2.1 Reaction-Induced Mass Discrimination (RIMD) Within Q2

The rf amplitudes of Q1, Q2, and Q3 are characterized here by the Mathieu parameters q1, q2, and q3, respectively (for further information, see references [2]–[5]). In this discussion and are used to represent, respectively, the values of q2 which correspond to the maximum ion transmission through Q2Q3 for the reactant (projectile) ion of mass mreact and for each product ion of mass mprod. The subscripts “react” and “prod” designate, respectively, the reactant ion A+ and the product ion C+ (or D+, etc.).

Within Q2, mprod/mreact=qreact/qprod [9]. Therefore, low-mass daughters are not detected when mprod/mreactq2>0.908 [5]. This means that the signal for each product ion must be measured at its respective . For each product ion, its corresponding {[A+]0 − [A+]} must also be measured at that same . That is, with reference to eq (4’), ∝ must be determined by measuring [C+] and {[A+]0 − [A+]} at the for C+; with reference to eq (5’), β must be determined by measuring [D+] and {[A+]0 − [A+]} at the for D+; etc. This must be done to compensate for the differences in ion containment efficiencies within the Q2Q3 structure.

For example, consider the CAD of a parent ion of m/z 196 which fragments to progeny ions of m/z 15, m/z 42, and m/z 86 [5]. In this case oc must be determined by measuring [15+] and [196+]0 − [196+]} at the for 15+; β must be determined by measuring [42+] and {[196+]0 − [196+]} at the for 42+; etc. The reader may wish to consult reference [5] for a more detailed exposition.

In principle, if XQQ instruments were well behaved, all measurements could be made at the for the progeny ion of lowest mass (m/z 15 for the example above [5]). However, because of ion-optical imaging (focusing) problems within the Q2Q3 structure, one usually observes oscillations in the ion intensity as q2 is varied [5]. Consequently, measurements of [C+], [D+], etc. and their respective {[A+]0 − [A+]} must be made at the local maxima closest to the of each product ion.

3.2.2 Intrinsic Mass Discrimination (IMD) within Q3

The resolution controls for Q3 must be varied as necessary to compensate for the mass discrimination intrinsic to Q3 {cf. [2]: p. 100, p. 105 (fig. 5.9), and p. 143–144}.

3.2.3 The Kinetic Energy of Product Ions Entering Q3

It is now recognized that in the LAB frame of reference the translational energy of product ions Eprod is generally related to the collision energy of the projectile (reactant) ion Ereact by: where n ≃ 1 − 2 for CAD processes studied to date [8]. Hence, if mprod/mreact < < 1, low-mass daughters exiting Q2 will have low kinetic energies. Consequently, the Q3 rod offset (pole bias) must be set sufficiently low with respect to the Q2 rod offset so that no low-mass, low-energy product ions are denied entry to Q3 by the Q3 potential barrier. On the other hand, the Q3 potential barrier must be sufficiently high to provide adequate resolution within Q3 [8]. The reader may wish to consult references [7] and [8] for a more detailed exposition with reference to the selection of the appropriate Q3 rod offset.

4. Kinetics-Based Protocol for MS/MS Measurements

In this section we detail the generic protocol used to measure instrument-independent CAD spectra in XQQ instruments. The reader is referred to the caveats detailed in items 1–9 of the Precautions in Appendix 1.

The protocol of Appendix 1 (for the NIST-EPA International Round Robin) must first be used to validate that any particular XQQ instrument can provide a dynamically-correct (i.e., instrument-independent) representation of ion-molecule reactions. If the instrument cannot pass the protocol (i.e., it cannot provide a dynamically-correct representation), one should not proceed any further until one has eliminated the ion-optical defect(s) so that the instrument can pass the protocol. If the instrument can provide a dynamically-correct representation, then the measurements for Part 1 of the round-robin protocol of Appendix 1 will provide an in situ calibration of the target thickness of the XQQ instrument.

For all MS/MS measurements, one must use a gas target thickness within the single-collision regime [<2 cm-mtorr (<0.267 cm-Pa) for Ar]. For CAD, use an Ar gas target since it has been demonstrated [22] that the equivalent excitation energy (i.e., the equivalent internal energy imparted to a parent ion) is significantly less for a polyatomic target than it is for a monatomic target. However, one may also use other inert gas targets (He, Ne, Kr, or Xe) as necessitated by the collisional energy requirements since it has been demonstrated that the equivalent excitation energy is the same for the inert gases at any given center-of-mass energy E [22]–[24] {E = ELAB [m2/(m1+m2)], where m1 and m2 are, respectively, the masses of the reactant ion and of the gas target}.

One must use a fixed operating voltage for the ion detector (multiplier, Daly detector, etc.). The fixed voltage should provide the best compromise between the signal-to-noise ratio (S/N) of A+ and the S/N of C+, D+, E+, etc.

One must make Conversion Gain measurements if an instrument uses analog current measurements. Conversion Gain is the ratio of the electron current output from an electron multiplier relative to the ion current input. The Conversion Gain measurements are used to correct for differences in mass-dependent detector response. If the instrument uses true ion pulse counting, Conversion Gain measurements are not needed [i.e., ignore instruction (d) and continue with instruction (e)].

Warning: Some instruments use analog current measurements, but report the ion intensities as equivalent ion count rates within their data systems. Such instruments still require Conversion Gain measurements.

Note: If possible, one should adjust the Conversion Gain of the ion detector so that one attains a mass-independent detector response. That is, so that the Conversion Gain is the same for a projectile ion and for its lowest-mass product ion (e.g., for CAD) or highest-mass product ion (e.g., for neutral gain reactions). This might be accomplished, for example, by adjusting the operating voltage of a high-voltage (e.g., 20–30 kV) conversion dynode which is used in conjunction with an electron multiplier. If it is not possible to attain a mass-independent detector response, then one must make corrections for difference in Conversion Gain for each parent ion and for each product ion.

To measure Conversion Gains, one may use a procedure analogous to that used for instructions 30–39 in Part 2 of the NIST-EPA round robin protocol in Appendix 1. However, to ensure that the most reliable correction factors are obtained, the mass-dependent Conversion Gain measurements would have to be made under actual operating conditions (viz., with the conversion dynode and electron multiplier at high voltages). This might be done by measuring the ion current striking the conversion dynode and relating it to the output current from the electron multiplier. One would have to demonstrate, however, that there is a one-to-one correspondence between the ion current input and the electron current output, independent of ion structure.

Note: For the rest of the instructions which follow, it is presumed that the ion signals correspond to a mass-independent detector response. That is, if necessary, the ion signals have been corrected for differences in Conversion Gain efficiency.

Adjust the appropriate ion-optical elements (e.g., the Q1 rod offset for QQQ instruments) so that (i) [B]0> > [A+]0 and (ii) the energy spread of the projectile ions entering Q2 is minimized (≤3 eV for 50% of the ions). (Refer to item 3 of the Precautions from the NIST-EPA round robin protocol in Appendix 1.) Measure the Q2 stopping curve in a manner analogous to that prescribed in instructions 8–10 of Part 1 of the NIST-EPA round-robin protocol in Appendix 1.

From the optimum Q2 stopping curve measurements determine E50 (corresponds to the Q2 rod offset that stops 50% of the reactant ions). Then set the Q2 rod offset equal to E50 − E where E is the requisite collision energy (in LAB coordinates).

One must use only a “daughter-scan” mode. That is, Q1 is tuned to the peak position which corresponds to the maximum ion intensity at the m/z of the projectile ion while Q3 scans over the peak which corresponds to the m/z of a product ion. The q2 must be referenced to q1 (i.e., to qreact) rather than to q3 (i.e., to qprod). Use a scan window of ca. 4–10 amu so one can see the entire mass peak (baseline-to-baseline).

Note: To ensure consistent measurements for the duration of each experiment, make sure Q1 is staying tuned to the peak position for the projectile ion intensity; this can be done by varying the Q1 mass command to maximize the ion signal being viewed within the Q3 scan window. Use the same fixed operating voltage for the ion detector as was used for instruction 4 (c).

One then adjusts iteratively the resolution parameters for Q3, and the Q3 rod offset as necessary to approximate ∝ + β + γ + … ≃ 1.0 for the ∝, ß, …, etc. measured in concordance with the precepts detailed in sections 3.2.1–3.2.3. To optimize the precision of all measurements, ion signals should be measured in “back-to-back” units [cf. item 6 (iii) of the Precautions from the NIST-EPA round-robin protocol in Appendix 1]. For example, for a fixed [B]L measure:

[C+], [A+], and [A+]0 at the for C+;

[D+], [A+], and [A+]0 at the for D+; etc.

Note: One must make appropriate corrections for any background contributions to [C+], [D+], etc. which may be observed in the absence of added target gas. Such background contributions can arise from decompositions of A+ (to produce C+, D+, etc. via CAD or RIF [9]) occurring between the rear of Q1 and the front of Q3. Such background CAD or RIF may be induced by the interaction of A+ with plumes of gas emanating from the ion source.

In practice, instructions (b)–(h) provide IMD-free and RIMD-free estimates for the branching ratios of fragment ions, except for very minor fragment ions adjacent to major ions. The resolution is not adequate for obtaining dynamically-correct estimates for the branching ratios of very minor fragment ions adjacent to major ions. Consequently, the resolution must be increased for each group of ions which contains a minor ion adjaent to a major ion, and complementary measurements must be made for the branching ratios of minor fragment ions within each group. Each group contains only fragment ions which are in close proximity (e.g., m/z 13–15), so that the measurements for each ion in the group are still IMD-free and RIMD-free within that group. To relate these individual group measurements to the overall CAD spectrum [viz., to the entire range of masses from the m/z of the reactant (parent) ion down to the m/z of the lowest-mass product ion], the measurements for each group must be referenced to the IMD-free and RIMD-free estimate for the branching ratio of a major fragment ion within that group. For example, for ECM≃2−40 eV, the CAD of C2H3O+ (m/z 43) from biacetyl produces fragment ions at the following m/z: 13, 14, 15, 26, 27, 28, 29. The fragment ions at m/z 13 and 14 are minor compared to the major fragment ion at m/z 15 (refer to table 1). Therefore, m/z 13–15 are measured under well-resolved conditions and referenced to the IMD-free and RIMD-free estimate for the branching ratio of the fragment ion at m/z 15. Similarly, the fragment ions at m/z 27 and 28 are referenced to the fragment ions at m/z 26 and/or 29.

Repeat (b)–(i) as necessary to accomplish ∝ + β + γ + …≃1.0±0.1 for each ECM, as necessitated by the reaction dynamics.

5. Proposed Format for MS/MS Database

Table 1 shows a typical CAD spectrum (measured with the protocol of sec. 4 [16]) in the format proposed for presentation of CBRIS.

5.1 Advantages

The advantages of this CBRIS database format are:

The partial cross sections ∝σ, βσ, etc. can characterize both known and unknown species (so long as the unknown species contain ionic substructures for which the CAD cross sections and product identities are known). Therefore, it may be possible to assign the structure of an unknown species on the basis of the absolute cross sections σij, σjk, etc. for the CAD of known ionic substructures i, j, k, etc. That is, this proposed use of CBRIS in MS/MS is analogous to the use of group frequencies in infrared spectroscopy.

Characterization of an unknown compound by using CBRIS does not require that the compound be in the CBRIS database. By contrast, to characterize an unknown by spectral matching within a “library”, the compound must usually be in the library. In this regard, development of a database of CBRIS for all substructures (or a subset thereof) would be very much more tractable than the development of a library of CAD spectra for all the source compounds that contain all the substructures (or a subset thereof). For example, consider the simplistic analogy where compounds correspond to words, and substructures correspond to letters: more than 500,000 words can be composed with only 26 letters of the alphabet.

The format is compatible with its use in expert systems. This should facilitate rapid real-time analysis of unknowns within computer-controlled field instruments.

End users are involved directly in its evolution by using critically evaluated cross sections already in the database and by submitting new cross sections for inclusion in the database.

6. Conclusions

A kinetics-based measurement protocol can provide accuracy and precision for CAD measurements within XQQ tandem mass spectrometers. This protocol can be used to develop a dynamically-correct (i.e., instrument-independent) MS/MS database (or library) for XQQ instruments.

[40Ar+]0	______	______	______	______	______	______	______	______
Q2R0, V	______	______	______	______	______	______	______	______
[40Ar+]0	______	______	______	______	______	______	______	______
Q2R0, V	______	______	______	______	______	______	______	______
[40Ar+]0	______	______	______	______	______	______	______	______
Q2R0, V	______	______	______	______	______	______	______	______

P/XQQ = ______	ln Y = ______	ln W = ______
	ARLEFF/LNY = ______	ARLEFF/LNW = ______

“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ=______		In Y = ______		In W = ______
		ARLEFF/LNY = ______		ARLEFF/LNW =______

“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ = ______		ln Y = ______		ln W = ______
		ARLEFF/LNY = ______		ARLEFF/LNW = ______

“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ = ______		ln Y = ______		ln W = ______
		ARLEFF/LNY = ______		ARLEFF/LNW = ______

“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ = ______		ln Y = ______		ln W = ______
		ARLEFF/LNY = ______		ARLEFF/LNW= ______

“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ = ______		ln Y = ______		ln W = ______
		ARLEFF/LNY = ______		ARLEFF/LNW = ______

[N2+]0	______	______	______	______	______	______	______	______
Q2R0, V	______	______	______	______	______	______	______	______
[N2+]0	______	______	______	______	______	______	______	______
Q2R0, V	______	______	______	______	______	______	______	______
[N2+]0	______	______	______	______	______	______	______	______
Q2R0, V	______	______	______	______	______	______	______	______

[N2+]0	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______
[N2+]0	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______
[N2+]0	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______

[N2+]	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______
[N2+]	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______
[N2+]	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______

[SF5+]	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______
[SF5+]	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______
[SF5+]	______	______	______	______	______	______	______	______
q2	______	______	______	______	______	______	______	______

“18.”______	“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ = ______

“18.”______	“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ = ______

“18.”______	“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ = ______

“18.”______	“19.”______	“20.”______	“21.”______	“22.”______	“23.”______
P/XQQ = ______