www.spec t roscopyo n l i n e . com

The interface region is probably themost critical area of the whole induc-tively coupled plasma mass spec-trometry (ICP-MS) system. It cer-tainly gave the early pioneers of the

technique the most problems to over-come. Although we take all the benefitsof ICP-MS for granted, the process of tak-ing a liquid sample, generating an aerosolthat is suitable for ionization in theplasma, and then sampling a representa-tive number of analyte ions, transportingthem through the interface, focusingthem via the ion optics into the massspectrometer, finally ending up with de-tection and conversion to an electronicsignal, are not trivial tasks. Each part ofthe journey has its own unique problemsto overcome but probably the most chal-lenging is the movement of the ions fromthe plasma to the mass spectrometer.Lets begin by explaining how the ion-

sampling process works, which will givereaders an insight into the many prob-lems faced by the early researchers.

SAMPLING THE IONSFigure 1 shows the proximity of the inter-face region to the rest of the instrument.The role of the interface is to transportthe ions efficiently, consistently, and withelectrical integrity from the plasma,which is at atmospheric pressure (760Torr), to the mass spectrometer analyzerregion, which is at approximately 1026Torr. One first achieves this by directingthe ions into the interface region. The in-terface consists of two metallic coneswith very small orifices, which are main-tained at a vacuum of ;2 Torr with a me-chanical roughing pump. After the ionsare generated in the plasma, they passthrough the first cone, known as the sam-pler cone, which has an orifice diameter

of 0.81.2 mm. From there they travel ashort distance to the skimmer cone,which is generally sharper than the sam-pler cone and has a much smaller orifice(0.40.8 mm i.d.). Both cones are usuallymade of nickel, but they can be made ofmaterials such as platinum that are farmore tolerant to corrosive liquids. To re-duce the effects of the high-temperatureplasma on the cones, the interface hous-ing is water-cooled and made from a ma-terial that dissipates heat easily, such ascopper or aluminum. The ions thenemerge from the skimmer cone, wherethey are directed through the ion optics,and finally are guided into the mass sepa-ration device. Figure 2 shows the inter-face region in greater detail; Figure 3shows a close-up of the sampler andskimmer cones.

CAPACITIVE COUPLING This process sounds fairly straight-forward but proved very problematic dur-ing the early development of ICP-MS be-cause of an undesired electrostatic(capacitive) coupling between the loadcoil and the plasma discharge, producinga potential difference of 100200 V. Al-though this potential is a physical charac-teristic of all inductively coupled plasmadischarges, it is particularly serious in anICP mass spectrometer because the ca-pacitive coupling creates an electrical dis-charge between the plasma and the sam-pler cone. This discharge, commonlycalled the pinch effect or secondary dis-charge, shows itself as arcing in the re-gion where the plasma is in contact withthe sampler cone (1). This process isshown very simplistically in Figure 4.

If not taken care of, this arcing cancause all kinds of problems, including anincrease in doubly charged interferingspecies, a wide kinetic energy spread ofsampled ions, formation of ions gener-

TUTORIALTUTORIALS P E C T R O S C O P Y

A Beginners Guide to ICP-MSPart IV: The Interface Region

R O B E R T T H O M A S

Iondetector

MSinterface

ICP torchIon optics

Spraychamber

Nebulizer

Mass separationdevice

Turbomolecularpump

Turbomolecularpump Mechanical

pump

Radio power supply

Figure 1. Schematic of an inductively coupled plasma mass spectrometry (ICP-MS) system,showing the proximity of the interface region.

26 SPECTROSCOPY 16(7) JULY 2001

ated from the sampler cone, and a de-creased orifice lifetime. These problemswere reported by many of the early re-searchers of the technique (2, 3). In fact,because the arcing increased with sam-pler cone orifice size, the source of thesecondary discharge was originallythought to be the result of an electro-gas-dynamic effect, which produced an in-crease in electron density at the orifice(4). After many experiments it was even-tually realized that the secondary dis-charge was a result of electrostatic cou-pling of the load coil to the plasma. Theproblem was first eliminated by ground-ing the induction coil at the center, whichhad the effect of reducing the radio fre-quency (RF) potential to a few volts. Thiseffect can be seen in Figure 5, taken fromone of the early papers, which shows thereduction in plasma potential as the coil isgrounded at different positions (turns)along its length.

Originally, the grounding was imple-mented by attaching a physical ground-ing strap from the center turn of the coilto the interface housing. In todays instru-mentation the grounding is achieved in anumber of different ways, depending onthe design of the interface. Some of themost popular designs include balancingthe oscillator inside the circuitry of theRF generator (5); positioning a groundedshield or plate between the coil and theplasma torch (6); or using two interlacedcoils where the RF fields go in opposingdirections (7). They all work differentlybut achieve a similar result of reducing oreliminating the secondary discharge.

ION KINETIC ENERGYThe impact of a secondary discharge can-

not be overestimated with respect to its ef-fect on the kinetic energy of the ions beingsampled. It is well documented that the en-ergy spread of the ions entering the massspectrometer must be as low as possible toensure that they can all be focused effi-ciently and with full electrical integrity bythe ion optics and the mass separation de-vice. When the ions emerge from the ar-gon plasma, they will all have different ki-netic energies based on their mass-to-change ratio. Their velocities should all besimilar because they are controlled byrapid expansion of the bulk plasma, whichwill be neutral as long as it is maintained atzero potential. As the ion beam passesthrough the sampler cone into the skim-mer cone, expansion will take place, but itscomposition and integrity will be main-tained, assuming the plasma is neutral.This can be seen in Figure 6.

Electrodynamic forces do not play arole as the ions enter the sampler or theskimmer because the distance overwhich the ions exert an influence on eachother (known as the Debye length) issmall (typically 10231024 mm) com-pared with the diameter of the orifice(0.51.0 mm) (8), as Figure 7 shows.

It is therefore clear that maintaining aneutral plasma is of paramount impor-tance to guarantee electrical integrity ofthe ion beam as it passes through the in-terface region. If a secondary dischargeis present, it changes the electrical char-acteristics of the plasma, which will affectthe kinetic energy of the ions differently,depending on their mass. If the plasma isat zero potential, the ion energy spread isin the order of 510 eV. However, if a sec-ondary discharge is present, it results ina much wider spread of ion energies en-

tering the mass spectrometer (typically2040 eV), which makes ion focusing farmore complicated (8).

BENEFITS OF A WELL-DESIGNEDINTERFACEThe benefits of a well-designed interfaceare not readily obvious if simple aqueoussamples are analyzed using only one setof operating conditions. However, it be-comes more apparent when many differ-ent sample types are being analyzed, re-quiring different operating parameters. Atrue test of the design of the interface oc-curs when plasma conditions need to bechanged, when the sample matrixchanges, or when a dry sample aerosol isbeing introduced into the ICP-MS. Ana-lytical scenarios like these have the po-tential to induce a secondary discharge,change the kinetic energy of the ions en-tering the mass spectrometer, and affectthe tuning of the ion optics. It is therefore

S P E C T R O S C O P Y T U T O R I A L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Plasmatorch

Ionoptics

~2 Torrvacuum

SampleconeSkimmer

cone

RF coil

Figure 2. Detailed view of the interfaceregion.

Interfacehousing

Samplercone

Skimmercone

Figure 3. Close-up view of the sampler andskimmer cones. (Courtesy PerkinElmer Instruments, Norwalk, CT.)

Secondarydischarge

Figure 4. Interface area affected by sec-ondary discharge.

100

80

60

40

20

00.0 0.5 1.0 1.5

Load coil turns

Plas

ma po

tentia

l

2.0 2.5 3.0

n

n

n

n

n

n

n

Figure 5. Reduction in plasma potential asthe load coil is grounded at different posi-tions (turns) along its length.

JULY 2001 16(7) SPECTROSCOPY 27

28 SPECTROSCOPY 16(7) JULY 2001 www.spec t roscopyo n l i n e . com

critical that the interface groundingmechanism can handle these types ofreal-world applications, of which typicalexamples include The use of cool-plasma conditions. It isstandard practice today to use cool-plasma conditions (500700 W power and1.01.3 L/min nebulizer gas flow) tolower the plasma temperature and reduceargon-based polyatomic interferences

such as 40Ar16O, 40Ar, and 38ArH, in thedetermination of difficult elements like56Fe, 40Ca, and 39K. Such dramaticchanges from normal operating condi-tions (1000 W, 0.8 L/min) will affect theelectrical characteristics of the plasma. Running volatile organic solvents. Ana-lyzing oil or organic-based samples re-quires a chilled spray chamber (typically220 C) or a membrane desolvation sys-

tem to reduce the solvent loading on theplasma. In addition, higher RF power(13001500 W) and lower nebulizer gasflow (0.40.8 L/min) are required to dis-sociate the organic components in thesample. A reduction in the amount of sol-vent entering the plasma combined withhigher power and lower nebulizer gasflow translate into a hotter plasma and achange in its ionization mechanism. Reducing oxides. The formation of ox-ide species can be problematic in somesample types. For example, in geochemi-cal applications it is quite common to sac-rifice sensitivity by lowering the nebulizergas flow and increasing the RF power toreduce the formation of rare earth ox-ides, which can interfere spectrally withthe determination of other analytes. Un-fortunately these conditions have the po-tential to induce a secondary discharge. Running a dry plasma. Samplingaccessories such as membrane desolva-tors, laser ablation systems, and elec-trothermal vaporization devices are beingused more routinely to enhance the flexi-bility of ICP-MS. The major difference be-tween these sampling devices and a con-ventional liquid sample introductionsystem, is that they generate a dry sam-ple aerosol, which requires totally differ-ent operating conditions compared with aconventional wet plasma. An aerosolcontaining no solvent can have a dramaticeffect on the ionization conditions in theplasma.

Even though most modern ICP-MS in-terfaces have been designed to minimizethe effects of the secondary discharge, it

S P E C T R O S C O P Y T U T O R I A L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ion optics

Skimmer Sampler

760 Torr34 TorrExpansion ofion beam

Ion beamSampleaerosol

Figure 6. The composition of the ion beam is maintained, assuming a neutral plasma.

Interfacecone

Debyelength

Orificediameter

Figure 7. Electrodynamic forces do not af-fect the composition of the ion beam enter-ing the sampler or the skimmer cone.

Circle 17

Tutorial continued on page 34

30 SPECTROSCOPY 16(7) JULY 2001 www.spec t roscopyo n l i n e . com

In this months column we will explorewhy the user requirements specifica-tion (URS) and the validation plan areso important for the validation of spec-trometry software, and well cover the

specification and system selection from asoftware perspective.

In the first installment of this series, welooked at the system development life cy-cle (SDLC) and some validation concepts(1). One concept was that validation is aprocess that covers the entire system de-velopment life cycle: Once started, youcant stop. Now we will look in more de-tail at the first part of the SDLC.

THE WAY IT WASIn the past, the spectrometer and soft-ware were purchased and then, just be-fore they were put into operational use,someone thought about validation. Somecommon questions may have been Have we validated the system? No. Does it matter? Probably. Will we get caught? Dont even think

about answering no to this question.Considering validation at such a late

stage of the life cycle will mean a delay in

going operational, thus failing to gainbenefit from the investment in the instru-ment or going live with no regulatorycoverage. It depends on your approach torisk and if can you sleep at night.

THE WAY IT SHOULD BEHowever, as we discussed in the previousarticle in this series, a proactive approachto validation is necessary and, if doneright, will actually save you money by en-suring that you buy the right instrumentfor the job. So well start at the beginningand look at the first stages of the life cycle: Defining and controlling the validation

throughout the whole life cycle (writ-ing the validation plan).

Specifying what you want the systemto do (writing a user requirementsspecification).

Selecting the system using the require-ments defined in the URS as the basis,rather than the salesperson boughtme a good meal.Defining and controlling the overall valida-

tion. The validation plan is one name forthe document that controls the validationeffort for your spectrometer software.However, the name for this documentvaries from laboratory to laboratory. Itmay be called the validation plan, mastervalidation plan, validation master plan, orquality plan.

Regardless of what you call this docu-ment in your organization, it should coverall the steps you are going to take todemonstrate the quality of the spectrome-try software in your laboratory.

Ideally the validation plan should bewritten as early as possible in the life cy-cle to define the overall steps that are re-quired as well as the documents to be pro-duced during each phase of the life cycle.There are different approaches to writingvalidation plans, and the document can bewritten in several stages in the life cycle.

Ill outline my philosophy and rationalenow and you, dear reader, can accept thisas is, modify it, or ignore it.

First, you should write the validationplan as either the first or second docu-ment in the life cycle; I advise writing it af-ter the first or second draft of the URS toincorporate any implementation or roll-outissues in the overall validation strategy.The rationale for this approach is that thevalidation plan provides documented evi-dence of intent of the validation. The doc-ument will set out the overall strategy ofthe validation and define the life cyclephases and the documented evidence thatwill be produced in each phase. If youleave writing the validation plan until laterin the project, one or more phases of thelife cycle will have passed and you mayneed to write documents retrospectively.Furthermore, youll be out of compliancewith 21 CFR 11.10(k)(2), which requiresa time-sequenced audit trail of systemsdocumentation.

Content of a validation plan. The purposeof a validation plan is to provide docu-mentation of intent for the whole valida-tion, including a definition of the life cycleused, documentation to be produced dur-ing the each stage of the life cycle, androles and responsibilities of everyone in-volved in the project.

To provide a better perspective, thecontent of a validation plan is listed in thesidebar. It is based on the Institute ofElectronic and Electrical Engineers(IEEE) standard for validation and verifi-cation plans (2).

This document is important because itdefines what you will do in the validation,and you will be judged against it whenyour operation is inspected. Therefore,read and understand it well dont writethe plan and forget it, because what youplan does not always come to pass. Usu-ally deviations from the plan occur that

F O C U S O N

QUALITYQUALITYQUALITYQUALITY

Validation of Spectrometry SoftwarePart II: Roles of the Validation Plan and User Requirements Specification

R . D . M C D O W A L L

Bob McDowallis a visiting seniorlecturer in theDepartment ofChemistry at theUniversity ofSurrey, principalof McDowallConsulting

(Bromley, UK), and Questions of Qualitycolumn editor of LCGC Europe,Spectroscopys sister magazine. Addresscorrespondence to him at 73 MurrayAvenue, Bromley, Kent BR1 3DJ, UK.

JULY 2001 16(7) SPECTROSCOPY 31

youll need to record, such asdocuments not written, new doc-uments required that have notbeen specified, or parts of thelife cycle omitted or modified.These changes will need to benoted under the deviation proce-dure that you have in place inthe plan. Noting the changessounds like a pain, but once theprinciples are understood, it isrelatively simple to do.

DESIGN: THE URSHow much money have youwasted on purchasing spectrom-eters that were not fit for pur-pose, did not do the job youwanted, or used software thatwas not up to snuff? From a busi-ness perspective, a documentthat says what you want the in-strument and software to do willbe beneficial, because youllhave a better chance of selectingthe right instrument andsoftware.

From a regulatory perspec-tive, remember that the defini-tion of validation presented inthe first part of this series (1) in-cluded that phrase predefinedspecifications. The documentthat provides the laboratory withthe predefined specifications forthe spectrometer and the soft-ware is the URS. Without thisdocument or an equivalent, youcannot validate your spectrometer soft-ware, because you dont have a prede-fined specification and therefore there isnothing to test against. This is particu-larly important when you consider whichelectronic record and electronic signa-ture functions are pertinent to define andtest for the way that you will use theinstrument.

The URS provides the answer to thequestion, What do you want the system todo? This makes the assumption that youknow what you want the system to do.

A well-written URS provides severalspecific benefits. For one thing, it servesas a reference against which off-the-shelfcommercial products are selected andevaluated in detail and any enhancementsare defined. Also, you are less likely to beseduced by technology or buy a poor sys-tem. Furthermore, the URS reduces thetotal system effort and costs, becausecareful review of the document should re-

veal omissions, misunderstandings, andinconsistencies in the specification. Thismeans that they can be corrected easilybefore you purchase the system. Finally,a well-written URS provides the input touser acceptance test specifications andqualification of the system.

General guidelines for a URS. A user re-quirements specification clearly and pre-cisely defines what the customer (that is,you) wants the system to do, and itshould be understood by both the cus-tomer and the instrument vendor. TheURS is a living document and must be up-dated, via a change control procedure,throughout the computer system life cy-cle. After purchase, when you upgradethe software, also update the URS to re-flect the changes and new functions inthe latest version.

A URS defines the functions to be car-ried out, the data on which the systemwill operate, and the operating environ-

ment. Ideally, the emphasis ison the required functions andnot the method of implementa-tion, as this may be the identifi-cation of a solution. The aim ofa URS is to make a statement ofrequirements rather than astatement of a potential solu-tion. This allows users to lookobjectively at software from dif-ferent vendors and make an ob-jective decision as to which sys-tem is required.

Nature of the URS. The URSshould address the followingbasic issues: Functionality: What is the sys-tem or function supposed to do? External interfaces: Howdoes the system interact withother systems and the users? Performance: What are thespeed, availability, and responsetime of the various functions ofthe system? Attributes: What are the secu-rity considerations of eachfunction? Design constraints: Must thesystem work on specific hard-ware or use an operating sys-tem, and are these consistentwith your organizationsstandards? Prioritization: All require-ments are ranked for impor-tance as either mandatory ordesirable (respectively, you

must use the system, or it would sim-ply be nice to have it).The URS should form the basis of the

solution to be delivered by the selectedvendor. If this does not happen, you canleave yourself open to a poor-quality prod-uct because either you dont know whatyou want the system to do or you cant ar-ticulate this need to the vendor.

Writing the specification. The followingguidelines should be followed during theproduction of the specification: Each requirement statement should be

uniquely referenced and no longerthan 250 words.

The URS should be consistent; there-fore, requirement statements shouldnot be duplicated or contradicted.

The URS should express requirementsand not design solutions.

Each requirement should be testable(this allows the tests to be designed assoon as the URS is finalized).

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Validation Plan Outline FormatBased on IEEE Std. 1012-1986

PurposeWhat is the scope of the validation: What is the spectrom-etry system to be validated?

Reference documentsInclude references to any regulations, documents, guide-lines, or internal policies and procedures that affect thevalidation of this spectrometer.

DefinitionsDefine key words and terms.

Validation overviewDefine the roles and responsibilities for the validation, andinclude both internal and external people involved.Meaning of signatures: Why are you signing a document?Cross-reference to the project plan for the schedule ofwork. (This should be a separate document and can beupdated as the validation progresses or not, as thecase may be.)

Life cycle validationDefine the system development life cycle that youll be us-ing for the validation. For each phase, state what the activ-ities will be and what documented evidence you will beproducing. Dont forget: some of this may be electronic,especially during the qualification phases.

Validation reportingOutline how the validation will be reported.

Validation administration proceduresState how change control for deviations, software bugs,and so forth will be handled. The validation plan is a con-trolled document, so it must be paginated correctly (for ex-ample, page X of Y), signed by the author, authorized bytwo others (technical and compliance/release reviews),and distributed to specified individuals.

32 SPECTROSCOPY 16(7) JULY 2001 www.spec t roscopyo n l i n e . com

Both customer and vendor must under-stand the document; therefore, jargonshould be avoided, and key wordsshould be defined in a specific sectionin the document.

Requirements should be prioritized asmandatory or desirable.

The URS should be modifiable, butchanges should be made under a for-mal control procedure.A URS is correct if every requirement

stated has only one interpretation and ismet by the system. Unfortunately, this israre.

Organizing requirements: Go with the work-flow. A URS can be extensive unless youplan well, so careful consideration shouldbe given to organizing requirements inthe easiest manner to understand. Thebest framework for writing a user re-quirements specification for most spec-trometers is to follow the process orworkflow that the data system will beautomating. Therefore, if you havemapped the process, it makes an idealprompt for the URS because the require-ments can be defined against each activ-ity in the process.

This idea of documenting what wewant in sufficient detail sounds great, butit means more work, doesnt it? Yes, thatis true, but consider the benefits. The

more time you spend in the specificationand design phase getting your ideas andconcepts right, the quicker the rest of thelife cycle will go if you know what youwant. You will get a spectrometer and as-sociated software that meet your require-ments more fully, and there will be lesschance later in the life cycle of finding outthat what looked good early on does notmeet certain key requirements now.

Contrast this to selecting a spectrome-ter with no user requirements. (This bitshould be easy, because we have all doneit.)

Dont forget the instrument specs! In thisseries well concentrate on the softwareelements, but dont forget the instrumentitself. The software and the instrumentmust be an integrated system. So, the in-strument specification also needs to beincluded in the overall URS. What operat-ing requirements do you need from thespectrometer, such as mass range andresolution or wavelength? Get them downin the URS.

A specific example. Table I shows an ex-ample of what a URS could look like. Itdefines the requirements for audit trailfunctionality in the spectrometer softwareto meet Part 11 requirements. Looks im-pressive, doesnt it? Look at the table andyoull see that each requirement is

uniquely numbered (not bad), short(good), and prioritized (getting better).However, 21 CFR 11 states that everychange must not overwrite the originalresult and must include the name of theuser, along with date and time of thechange. This is not mentioned in thisspecification (bummer!). So be careful,specify the system, and review it carefullyor something essential may be missed.

SYSTEM SELECTION: PART ONEBecause your requirements for the over-all system are contained in the URS, thedocument can be used as a basis todesign the tests to evaluate the varioussystems offered by vendors. Can the sys-tems offered meet your requirements, es-pecially for the mandatory functions? Us-ing the URS requirements for systemselection helps ensure that the system se-lected matches your business needs.

Dont forget that the tests you use forsystem selection should also include com-mon problems that you know happen inyour laboratory. What happens whensamples are switched and you notice theerror only after the analysis? Can the sys-tem handle the changes easily and withsuitable audit trail entries?

The system you select will be based onthe practical experience of using it inyour laboratory environment. However,before you sign on the dotted line, youmay want to make sure that the softwarewas developed in a quality mannerthrough a vendor audit.

VENDOR CERTIFICATES AND AUDITSMany spectrometer vendors will be certi-fied to ISO 9000 of some description andwill offer you a certificate that the systemconforms to its quality processes. This isfine, but please remember that no re-quirement for product quality exists inany ISO 9000 schedule, and if you look atthe warranty of any software product,there is no guarantee that the software isstated to be either fit for purpose or errorfree. The certificates are fine, but if thesystem is critical to your operation, myadvice is to consider a vendor audit.

The vendor audit should take placeonce the product has been selected. Thepurpose is simply to see if the ISO 9000quality system is operated effectively. Theevaluation and audit process is a very im-portant part of the life cycle, because itshows whether design, building, and test-ing stages (which are under the control

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Table I: Example Specification of Audit Trail Requirements for Spectrometer Software

Requirement Spectrometer Data System PriorityNumber Feature Specification (M/D)*

5.1.01 The system software requires an audit trail to monitor the creation, modification and deletion of all electronic records generated and managed by the system. M

5.1.02 The audit trail covers all acquisition, control, calibration, calculation, display, reporting, and export functions and includes all file handling options such as open, copy, edit, rename, and delete. M

5.1.03 The audit trail is able to support the system during normal operation without an excessive system overhead or loss of performance. D

5.1.04 The audit trail once invoked cannot be switched off. M5.1.05 Archival of electronic records will have an audit trail entry. M5.1.06 Selected portions of the audit trail must be made available

either by printing or viewing. These partial audit trail reports must be made available in a portable electronic format for use by regulatory agencies. D

5.1.07 The audit trails must be maintained for as long as the electronic records they correspond to exist. M

5.1.08 When a record is changed, all previous versions must be readable or available for inspection. M

*Mandatory or desirable

34 SPECTROSCOPY 16(7) JULY 2001 www.spec t roscopyo n l i n e . com

of the vendor) have been checked to en-sure compliance with the regulations.The audit should be planned and shouldcover items such as the design and pro-gramming phases, product testing and re-lease, documentation, and support; a re-port of the audit should be produced afterthe visit. Two published articles have cov-ered vendor audits in more detail (3, 4).

The minimum audit is a remote vendoraudit using a checklist that the vendorcompletes and returns to you. This isusually easy to complete, but the writer ofthe checklist must ensure that the ques-tions are written in a way that can be un-derstood by the recipient, because lan-guage and cultural issues could affect aremote checklist. Moreover, there is littleway of checking the answers you receive.However, for smaller software systems and some spectrometers fall into this cat-egory a remote audit is a cost-effectiveway of getting information on how a ven-dor carries out its development process,so long as you know and understand itslimitations.

SYSTEM SELECTION: PART TWOIf the vendor audit, price quote, instru-ment, and software are all acceptable,youll be raising a capital expenditurerequest (or whatever it is called in yourorganization) and then generating a pur-chase order. The quote and the purchaseorder are a link in the validation chain;they provide a link into the next phase ofthe validation life cycle: qualification. Thepurchase order is the first stage in defin-ing the initial configuration of the system,as well discover in the next article in thisseries.

REFERENCES(1) R.D. McDowall, Spectroscopy 16(2),

3243 (2001).(2) IEEE Standard 1012-1986, Software Vali-

dation and Verification Plans, Institute ofElectronic and Electrical Engineers, Pis-cataway, NJ, USA.

(3) R.D. McDowall, Sci. Data Mgmt. 2(2), 8(1998).

(4) R.D. McDowall, Sci. Data Mgmt. 2(3), 8(1998). u

F O C U S O N Q U A L I T Y . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .

shouldnt be taken for granted that theycan all handle changes in operating con-ditions and matrix components with thesame amount of ease. The most notice-able problems that have been reportedinclude spectral peaks of the cone mater-ial appearing in the blank (9); erosion ordiscoloration of the sampling cones;widely different optimum plasma condi-tions for different masses (10); and in-creased frequency of tuning the ion op-tics (8). Of all these, probably the mostinconvenient problem is regular opti-mization of the lens voltages, becauseslight changes in plasma conditions canproduce significant changes in ion ener-gies, which require regular retuning ofthe ion optics. Even though most instru-ments have computer-controlled ion op-tics, it becomes another variable thatmust be optimized. This isnt a majorproblem but might be considered an in-convenience for a highsample through-put lab. There is no question that theplasma discharge, interface region, andion optics all have to be designed in con-cert to ensure that the instrument canhandle a wide range of operating condi-tions and sample types. The role of theion optics will be discussed in greater de-tail in the next installment of this series.

Tutorial continued from page 34

Circle 22, 23

REFERENCES(1) A.L. Gray and A.R. Date, Analyst 108,

1033 (1983).(2) R.S. Houk, V. A. Fassel, and H.J. Svec, Dy-

namic Mass Spectrosc. 6, 234 (1981).(3) A.R. Date and A.L. Gray, Analyst 106,

1255 (1981).(4) A.L. Gray and A.R. Date, Dynamic Mass

Spectrosc. 6, 252 (1981).(5) S.D. Tanner, J. Anal. At. Spectrom. 10,

905 (1995).(6) K. Sakata and K. Kawabata, Spectrochim.

Acta 49B, 1027 (1994).(7) S. Georgitus, M. Plantz, poster paper pre-

sented at the Winter Conference onPlasma Spectrochemistry, FP4, FortLauderdale, FL (1996).

(8) D.J. Douglas and J.B. French, Spec-trochim. Acta 41B, 3, 197 (1986).

(9) D.J. Douglas, Can. J. Spectrosc. 34, 2(1089).

(10) J.E. Fulford and D.J. Douglas, Appl. Spec-trosc. 40, 7 (1986).

Robert Thomas has more than 30 years ex-perience in trace element analysis. He is theprincipal of his own freelance writing andscientific consulting company, Scientific So-lutions, based in Gaithersburg, MD. He canbe contacted by e-mail at thomasrj@bellatlantic.net or via his web site atwww.scientificsolutions1.com.u