1 3 0 LCGC NORTH AMERICA VOLUME 27 NUMBER 2 FEBRUARV 2009 www.chromatographyonllne.coin

M S - T H E PRACTICAL ART

When a practice such asLC-MS becomes so diverseand prolific the termsphrases adopted fromvarious sources findinga way into the practiceneed scrutiny. This issuecompletes the multi-partMS primer with a listof terms derived fromcommon usage throughoutvarious industries and,while included with the MSPrimer, is intended to standalone.

A Mass Spectrometry Primer,Part IV

Michael P. BaloghMS The Practical Art Editor

This issue completes the multi-part MS primer with a list ofterms derived from commonusage throughout various industriesand, while included with the MSPrimer, is intended to stand alone. Anumber of current books recognize theneed to monitor and modify the lan-guage we use wichtn LC-MS. Whena practice such as LC-MS becomesso diverse and prolific the terms andphrases adopted from various sourcesfinding a way into the practice needscrutiny. Many books such as the"Mass Spectrometry Desk reference"(2nd edition, O. David Sparkman,paperback 198 pages published byGlobal View Publishing, June 30,2006) provide more than a descriptionof fundamentals making an attempt toexplain and unify usage.

Although I prefer IMMS there maybe little problem in most cases deter-mining that 'IMS' means ion mobilitymass spectrometry and not imagingmass spectrometry let alone standingas a reference to Dr. Jed Diamond'sirritable male syndrome (although I'msure cases could be made).

My thanks to everyone - I lookforward to the comments postedin the margin at www.Waters.com(Resource Library/Primers). The webversion and the hardcopy (due out inearly 2009) have clearly benefit fromyour contributions.

IGlossaryThe following list of terms is derivedfrom common usage throughout theindustry as an adjunct to the discus-sions in this primer and includesterms and techniques no longer incommon usage.

Abundance: When viewed assimilar to absorbance displayed on aUV detector, the vertical increase insignal above background indicates anincreased occurrence ot that particularion (when the x axis is calibrated inmass units) or total ions present (whentbe horizontal axis is calibrated in timeor scans), The signal for all ions result-ing from the fragmentation of a singleanalyte or compounds compared to abase peak (the relative abundance ofeach ion) is used ro determine the fitof a fragmented pattern to a libraryspectrum for positive identification.

Accurate mass: The measured massvalue for a compoLind with an associ-ated error like 5 ppm. Accurate massalso is used commonly to refer to thetechnique rather than the measuredmass. Exact mass is the exact theoreti-cal value for the mass of a compound.

Atmospheric solids analysis probe(ASAP): Based upon work by Horn-ing in the 1970s, this form of sampleionization developed by McEwen andMcKay uses a standard atmosphericpressure chemical ionization (APCI)plasma but forms ions by placing thesample in a heated nitrogen stream.The heat volatilizes a surprisinglylarge number of samples, and ionsare formed by charge exchange withmetastable ions created by the APCIplasma. Relatively unambiguous iden-tifications can be made of individualcompounds from complex mixtures atlow levels using accurate mass instru-ments. See also DART and DESI.

Atmospheric pressure ionization(API): The term used to refer gener-ally to techniques such as electrosprayionization (ESI) and APCI and othersthat operate at atmospheric pressure.

1 3 2 LCGC NORTH AMERICA VOLUME 27 NUMBER! FEBRUARY 2009 www.chromatogra phyonlin e. com

MARKESmarKes-usa.

Attention GC-MSusers

^ accessories &onsumables catalogr thermal desorption

fact Markes now for'-lur FREE copy

ne shopnow open

Visit for all your TDspares &. consumables

http://shop.markesus.com

focusiT: (866) 483-5684 (toll free)

E; enquiries@markes.com

Atmospheric pressure chemicalionization (APCI)i Originally calledsolvent-mediated electrospray, it isapplied successfully more often to neu-tral molecules that do not ionize easilydirectly out of solution. APCI providesa current on a sharp pin, positioned inthe on-coming aerosol stream, to createa plasma of metastable ions from thesolvent itself and transfer the chargefrom chesc ions to the analyte as itpasses through the plasma. Heating aprobe through which the liquid chro-matography (LC) or solvent streampasses creates the aerosol.

Atmospheric gas chromatography:Developed by Charles McEwen atDuPont in 2002. Using a heated trans-fer line, a standard gas chromatogra-phy (GC) effluent can he introducedto a standard API (or ESl-APCI)source on a mass spectrometer. Thisprovides an easy and fast changeoverfrom ESI to gas chromatography (GC)for compounds chat would be best ana-lyzed hy GC. Mode of ionization canbe either APCI or atmospheric pressurephotoionization (APPI).

Atmospheric pressure photoion-ization (APPI): Developed in the1980s but commercialized after 2000when krypton gas lamps were foundto generate sufficient photon energyat 10 eV (approximately) to ionizenonpolar analytes such as PAHs andsteroids not typically amenable to ESIand APCI ionizarion.

Base peak: Usually the mostintense peak in the spectrum to whichothers are compared; in ionizationtechniques that give extensive struc-tural information such as electronionization (El), the base peak may notbe the parent or molecular Ion.

Calihration: Substances of knownmass are introduced usually as a constantflowing 5tream while the mass spectrom-eter software acquires a signal for a givenset of filtering conditions (that is, RF/DCratio for a quadrupole instrument). Aftercomparing the acquired signal to a refer-ence file, a calibration look-up table is cre-ated in the software. The calihration tableis then the basis for the mass-to-chargeratios passed hy the quads to be assigned aspecific value. See the MS Primer sectionson "Quantitation and Calibration."

Charge residue mechanism:Related to electrospray ionization;a theory first proposed in 1968 byMalcolm Dole in which he hypoth-esized that as a droplet evaporates,its charge remains unchanged. Thedroplet's surface tension, ultimatelyunable to oppose the repulsive forcesfrom the imposed charge, explodesinto many smaller droplets. TheseCoulombic fissions occur until dtop-lets containing a single analyte ionremain. As the solvent evaporatesfrom the last droplet in the reductionseries, a gas-phase ion forms.

Chemical ionization (CI): Colli-sions induced at low vacuum (0.4 torr)hy the introduction of a reagent for thepurpose of enhancing the productionof molecular ions and often sensitivity;as this is a much lower energy process[han electron impact ionization, frag-mentation is reduced and it is oftenreferred to as a soft ionization tech-nique. See electron ionization.

Collision-induced dissociation(CID): Also referred to as collision-ally activated dissociation (CAD), itis a mechanism by which molecularions are fragmented in the gas phasehy acceleration (using electricalpotential) to a high kinetic energyin the vacuum region followed hycollision with neutral gas moleculessuch as helium, nitrogen or argon. Aportion of the kinetic energy is con-verted or internalized by the colli-sion, which results in chemical bondsbreaking and the molecular ion isreduced to smaller fragments. Somesimilar "special purpose" fragmenta-tion methods include electron trans-fer dissociation (ETD) and electron-capture dissociation (ECD). See theMS Primer section on "Biomolecularionization methods."

Direct analysis real time (DART):Developed by Robert Cody and othersin 2002, similar to desorption electro-spray ionization (DESI) in applicationalthough more closely related to APCIin function. A sample is placed on asubstrate and bombarded by energizedparticles formed in a process similarto APCI. That is. metastable ions areformed by a plasma and transportedby heated nitrogen gas directed at the

1 3 4 LCGC NORTH AMERICA VOLUME 37 NUMBB2 FEBHUAfiY 2009 wvvw.chromatographyonllne.com

target. See also McEwen's work listedunder atmospheric GC and ASAP.

Delayed extraction (DE):Developed for matrix-assisted laserdesorption ionizarion time-of-flight(MALDl-TOF) instruments, it "cools"and focuses the ions for approxi-mately 150 ns after they form andbefore accelerating the ions into theflight tube. The cooled ions have alower kinetic-energy distribution thanuncooled ones, and they ultimatelyreduce the temporal spread of theions as they enter the TOF analyzer,resulting in increased resolution andaccuracy. DE is significantly lessadvantageous with macromolecules (forinstance ptoteins >30,000 Da).

Desorption electrospray ioniza-tion (DESI): First described by Gra-ham Gooks in 2002 as a means ofproducing soft secondary ions from(typically) an inert substrate surface.Analogous to MALDI using an ESIprobe aimed at about 50" incidentangle to the surface allowing ions tochemically sputter and be admitted tothe mass spectrometer. Shown to pro-

duce information directly from manypolar and nonpolar surface materials(skin, inract fruit for pesticide residuedetection, and so fotth) without theneed for sample preparation. See alsoMcEwen's work listed under armo-spheric GG and ASAP.

Desorption ionization on silica(DIOS): Once viewed as an alter-native to a preparing samples in aMALDI substrate, especially for smallmolecules because the substrate (abare silica surface) would not gener-ate interfering ions. Its commercialpotential in the late 1990s diminishedas the difficulties of producing theplates and the surface susceptibility tocontamination became apparent.

Direct current (DC): For our intet-ests, the term usually is used iti con-junction with "radio frequency" (RF)when describing how the quadrupolefunctions as a mass filter. Superim-posed RF and constant DG potentialsbetween four parallel tods were shownby Wolfgang Paul in 1953 to act as amass separator, or filter, where only ionswithin a particulat mass range, exhibit-

ing oscillations of constant amplitude,could collect at the analyzer.

Electron ionization (EI); Some-times incorrectly referred to as "elec-tron impact" ionization resulting fromthe interaction of an electron with aparticle (atom or molecule); can bethought of as a "hard" ionization tech-nique, as sufficient energy is impartedto disrupt internal chemical bondsrequiring high kcal/mol. Ionizing volt-age (typically 70 eV) refers to the dif-ference in voltage causing accelerationof the electrons used to induce elec-tron ionization. Unlike GI, EI avoidsuncontrolled collisions by operating athigh vacuum. The analyzer operatesat even higher vacuum (10"'' to 10"''totr).

Electrospray ionization (ESI): Aso-called "soft" ionization technique.The most widely employed ot the APItechniques. Gommercially significantsince the late 1980s, the phenomenonis attributed to an excess of energy(voltages in the 3-5 kV range) appliedto a conductive tube (stainless steelcapillary) inducing the liquid flowing

Everything for Headspace AnalysisSUN-SRi offers the most comprehensive range of headspace vials andclosures for total instrument compatibility. _Magnetic closures

Bi-metal for easy crimping -Screw thread

Unique septa i' SPME I

Liquid liquid extraction -Low background alcohol

Vial choices' Amber or dear glass' Sizesof6,10and20mL Flat or round base' Beveled edge for improved sealing |l

Get everything you need for headspace analysis. Visit www.sun-sri.com/everything or call800-522-8425 to request FREE samples and our latest catalog.

See our products at Pittcon, Booth #1125

1 3 6 tCCC NORTH AMEniCA VOLUME 27 NUMBER 2 FEBRUARY 2009 www.chromatographyQnHne.com

inside to exceed its Rayleigh limits andforming an aerosol upon exiting thetube. The resulting spray (the result ofa coulombic explosion} then gives riseto ions contained in the aerosol dtop-lets as they desolvate to approximatelya 10-|xm radius. The ions typically areprotonated and detected in the formM+H in positive ionization mode orMH in negative ion mode,

Elemental analysis: The nominalmass of an ion, molecule, or radical isthe sum of the nominal masses of theelements in its elemental composition.Achieving accurate mass measurementis based upon the calculated elementalcomposition but the term "elementalanalysis" is performed typically oninorganic materials to determineelemental makeup, not structure in some cases using solid metalsamples. Inductively coupled plasma(ICP) sources are common where adischarge (or lower power-glow dis-charge) device ionizes the sample.Detection using dedicated instru-ments, at the parts-per-trillion level,is not uncommon.

Exact mass: The exact theoreti-cal value for the mass of a compound."Accurate mass" is the measured massvalue for a compound with an associatederror like 5 ppm. Accurate mass also isused commonly to refer to the techniquerather than the measured mass.

Fast atom bombardment (FAB):One of the earlier so-called soft ioniza-tion techniques where the result is usu-ally an intense molecular ion with littlefragmentation. The analyte is placed ina matrix (often glycerol) either flowingor mote commonly placed on tbe tip ofa probe and positioned in the path ofhigh-energy atoms often xenon orcesium iodide. The technique has beeneffective for biomolecules up to 10,000amu, but perhaps mote importantly, inconjunction with the magnetic sectormass spectrometer, where exact weightcan be determined such as for novelpeptides. Sensitivity can be very good(low femtomole levels). The techniquecan be difficult to master, as tbe glyc-erol does foul the mass spectrometersource and the lower masses can beobscured by the presence of glycerol

ions. This technique is little used nowsince the introduction of ESI.

Field ionization (FI): The soft ion-ization provided by FI leads to littleor no fragmentation for a wide varietyof analytes. This is particularly sig-nificant in petrochemical applications,in which there are limitations to theutility of other ionization techniquesdue to fragmentation (EI) or complexionization chatacteristics (CI). A thinwire with a high applied voltage isheated in the vapor of an organic com-pound such as indene. The resultingstructures that are deposited on thesurface of the wire, dendtites, are pyto-lized, producing very fine conductivefilaments. When a very fine point hasa high potential applied to it, a veryintense electric field is generated at thistip, providing conditions under wbichfield ionization can take place.

Sample molecules pass in close prox-imity to the tips of a mass of catbondendtites grown on the EI emitter. TheFI emitter is positioned in close prox-imity to a pair of hollow extractionrods. The emitter Is held at groundpotential and a relatively high voltage(12 kV) is applied to the rods, produc-ing very high electric fields around thetips of the carbon dendrites. The GCcolumn is positioned in close proxim-ity and in line with the emitter wire.Under the influence of the electricfields, quantum tunneling of a valenceelectron from the molecule takes placeto give an ion radical.

Flow injection analysis (FIA):This is the practice of introducing asample (usually purified in an earlierstep, as a fraction to remove interfer-ences and complexity from the result-ing spectrum) through the LC injec-tor, but without a column in-line. TheLC system acts as a sample introduc-tion device only.

Filament: In EI the filament is thesource of electrons that interact withthe anatyte to ionize it. Typically madefrom a metal wire (flat or round) capa-ble of giving off 70-eV electrons as itbeats from current passing rhrough it.

Fragment ion: An ion produced asthe loss irom a parent molecular ion.The sum of the dissociated fragmentsequals the parent and under given con-

ditions will always fragment the sameinternal bonds to produce a predict-able pattern (same ions and relativeabundances fot each). See also product(daughter) ions resulting from specificMRM experiments.

Gas phase ion: Analytes must beconverted ftom their testing state toan ion in order to be manipulatedand acquired by a mass spectrometer.There are a number of ways to accom-plish this as described in the primer- some more aggressively creatingfragments while others conserve theanalyte intact. Energy is presented tothe analyte, producing an ion in thegas phase as opposed to, for instance,the condensed phase used to separateanalytes in LC.

Hybrid: Usually tefers to aninstrument that is a combination oftwo different types, for instance, ear-lier mictomass "hybrids" combinedmagnetic sector and quadrupoles.Today's quadrupole time-of-flight(QTOF) instrument is a hybrid ofquadtupole and TOF.

Ion: Mass spectrometers can onlymanipulate and therefore detect a masswhen it possesses at least one charge.When only one charge is present (forinstance, by the toss of an electron,causing the molecule to exist as a posi-tively charged cation radical or by theaddition of a proton or hydrogen toexist as a positively charged pseudo-molecular ion), we can think of it asrepresenting the molecular weight in alow-resolution scheme.

Ion current (total ion current):Tbe electtic current detected basedupon the charged particles createdin the ion source. If the mass spec-trometer is set to scan over a range of100-500 Da, tbe resulting total ioncurrent will be the sum of all ionspresent in the source within that rangeat tbe selected time. If the instrumentis set to detect only one ion (selectedion monitoring), the resulting total ioncurrent will be tbe sum of only thation at each selected instance.

Ion cyclotron resonance (ICR):Cyclotron instruments trap ions elec-trostatically in a cell using a constantmagnetic field. Pulses of RF voltagecreate orbital ionic motion, and tbe

1 3 8 LCGC NORTH AMERICA VOLUME 27 NUMBER 2 FEBRUARY 2009 www.chromatographyonline.com

orbiting ions generate a small signalat the detection plates ofthe cell (theion's orbital frequency). The frequencyis related inversely to the ions' mass-to-charge ratio (w/z), and the signalintensity is proportional to the numberof ions ofthe same m/z in the cell. Atvery low cell pressures, a cyclotroninstrument can maintain an ion's orbitfor extended periods, providing veryhigh resolution measurements. Fast-Fourier ttansform-ion cyclotron reso-nance (FT-ICR) instruments representthe extreme capability of measuringmass with the ability to tesolve closelyrelated masses. Although impracticalfor most applications, a 14.5-T magnetcan achieve a resolution of mote than3.5 million and, thus, display the dif-ference between molecular entitieswhose masses vary by less than themass of a single electron.

Ion evaporation mechanism: Ref-erence to ESI; in 1976, Iribarne andThomson proposed the ion evaporationmechanism (see also "charge residuemechanism"), in which small dropletsform by Coulombic fission, similarto the way they form in Dole's chargeresidue model. However, accordingto ion evaporation theory, the electricfield strength at the surface ofthedroplet Is high enough to make leavingthe droplet surface and transferringdirectly into the gas phase energeticallyfavorable for solvated ions.

Ion mobility: Tbis technique differ-entiates ions based upon a combinationof factors: their size, sbape, and charge.as well as their mass. Ion mobilitymeasurements and separations coupledwith tandem mass spectrometry (MS)can help overcome analytical chal-lenges that could not be addressed byother analytical means, including con-ventional MS or LC instrumentation.Ion mobility mass spectrometry (orIMMS because "imaging mass spec-ttometry" is often abbreviated IMS)devices are used commonly in airportsand handheld field units for rapidly(20 ms) detecting small moleculeswhose mobility is known. A wide rangeof analytes are amenable to IMMS.

Ion source: The physical space inthe ion stream in front ofthe analyzer,where the analyte is ionized. Each type

of interface requires its own internalgeometry for optimum results.

Ion trap: Also linear trap and Q-trap (or quadrupole trap). WolfgangPaul's invention ofthe quadrupole andquadrupole ion trap earned him theNobel Prize in physics in 1953. Anion trap instrument operates on prin-ciples similar to those of a quadrupoleinstrument. Unlike the quadrupoleinstrument, however, which filtersstreaming ions, the trap stores ionsin a three-dimensional space. Beforesaturation occurs from too many ionsattempting to fill the finite space,the trap or cyclotron allows selectedions to be ejected for detection. Fieldsgenerated by RF voltages applied toa stacked or "sandwich" geometry(end-cap electrodes at opposing ends)trap ions in space between the twoelectrodes. Ramping or scanning theRF voltage ejects ions from tbeir secu-lar frequency, or ttapped condition.Dynamic range is sometimes limited.The finite volume and capacity for ionslimits tbe instrument's range, espe-cially for samples in complex matrices.

Tbe ability to perform sequentialfragmentation and, thus, derive morestructural information from a singleanalyte (that is, fragmenting an ion,selecting a particular fragment, andrepeating the process) is called MS".GC peaks are not wide enough toallow more than a single fragmentation(MS-MS or MS-^ ). Ion-trap instru-ments perform MS-MS or fragmenta-tion experiments in time rather tbanin space, like quadrupole and sectorinstruments. So tbey cannot be usedin certain MS-MS experiments sucbas neutral loss and precursor ion com-parisons. Also, in MS-MS operationwirb an ion-trap instrument, tbe bot-tom tbird ofthe MS-MS spectrum islost, a consequence of trap design. Tocounter the loss, some manufacturersmake available via their software widerscan requirements that necessitate tbeswitching of operating parameters dur-ing data acquisition.

Because of similarities in functionaldesign, quadrupole instruments arehybridized to incorporate the advan-tages of streaming quadrupole and iontrapping behavior to improve sensitiv-

ity and allow on-tbe-fly experimentsnot possible with eitber alone. Sucbinstruments are sometimes called lin-ear traps or Q-traps). The increasedvolume of a linear trap instrument(over a three-dimensional ion trap)improves dynamic range.

Isotope ratio: Although oftenpresumed to be constant and stable,natural isotope abundance ratios showsignificant and characteristic varia-tions when measured very precisely.Isotope ratio measurements are usefulin a wide range of applications, forexample, metabolic studies using iso-topically enriched elements as tracers;climate studies using measurementsof temperature-dependent oxygen andcarbon isotope ratios in foraminifers;rock age dating using radiogenicisotopes of elements sucb as lead,neodymium or strontium; and sourcedeterminations using carbon isotoperatios (for instance to determine ifa substance occurs naturally or is apetroleum-based synthetic).

Typically, single tocusing magneticsector mass spectrometers with fixedmultiple detectors (one per isotope)are used. Complex compounds arereduced to simple molecules beforemeasurement for example, organiccompounds are combusted to CO2,H^O and Nj.

Laser ablation: Compounds can bedissolved in a material tbat acts as anintermediate to transfer charge to tbeanalytes of interest. A laser is aimed atthe mixture to cause sputtering to pro-duce ions in the space just above themixture, where tbey can be sampledor drawn into the mass spectrometer.Especially useful as a very soft ioniza-tion technique to look at intact largemolecules because the low-mass ionsfrom the matrix produce a highly com-plex, intense background tbat couldotherwise interfere with analytes ofsimilar low mass.

Magnetic sector: Ions leaving tbeion source are accelerated to a highvelocity; they then pass through amagnetic field perpendicular to tbeirdirection. When acceleration is appliedperpendicular to the direction ofmotion, rbe object's velocity remainsconstant, but tbe object travels in a

1 4 0 LCGC NORTH AMERICA VOLUME 27 NUMBER 2 FEBRUARY 20D9 www.chromatographyonllne.com

circular pach. Therefore, the magneticsector is designed as an arc. Ions witha constant kinetic energy but differentmass-to-charge ratio are brought intofocus at the detector slit (called the"collector slit") at different magneticfield strengths.

The magnetic sector alone willseparate ions according to their mass-to-charge ratio. Howevet, because theions leaving the ion source do not haveexactly the same energy (and thereforedo not have exactly the same veloc-ity), the resolution will be limited. Anelectric sector that focuses ions accord-ing to their kinetic energy usually wasadded, which, like the magnetic sector,applies a perpendicular force to thedirection of ion motion.

Matrix assisted laser desorptionionization (MALDI): First intro-duced in 1988 by Tanaka, Karas,and Hillenkamp, MALDI uses alaser to strike and energize a matrixcontaining the analyte. It has provento be the method of choice for ion-izing exceptionally large peptide andprotein molecules that can then bedetected intact. Commonly employedas the introduction scheme for TO Finstruments, which are often referredto as MALDI-TOF.

Mass-to-charge ratio (m/z):Charged particles are represented asa ratio of their mass to their ioniccharge. In literature and general use,this often appears as m/z, where theanalyte from which the ion is derivedmight be labeled using atomic massunits (amu), daltons or molecularweight (mw).

Mean free path: The distance fromentrance of an ion into the analyzerand detection ofthat ion. At operatingvacuum, the mean free path is rela-tively long considering time betweencollisions in ratified air versus the timeneeded to analyze an ion. For example:

atm (1000 totr) air contains3 X 10^ ^ molecules/cm^

chamber at 1 X 10"'' torr contains3 X 10" molecules/cm'

\ V pressure (torr) minimummean free path (cm)

where \ = 5 X 10"' cm.Molecular ion: The ion produced

when a molecule gains (anion) or loses

(cation) an electron. See also pseudo-molecular ion.

Monoisotopic mass: The monoiso-topic mass of an element is the exactmass of the most abundant, naturallyoccurring, stable isotope. In a massspectrometer able to report only to thenearest integer value, the molecular ionof a C.gHjQj compound might be rep-resented by a peak at m/z 703 insteadof at m/z 702, because the molecularion would have a monoisotopic mass of702.7825, which rounds to the integer703. Once an ion exceeds a nominalmass of 1000 Da, there is no observednominal m/z value peak in the massspectrum. The monoisotopic masspeak is offset from where the nominalmass peak should be observed by anamount equal to the mass defect of theion. For single-charge ions with massesabove 500 Da, using techniques likeelectrospray with transmission quad-tupole or quadrupole ion-trap massspectrometers that have unit resolutionthroughout rhe m/z scale, the isotopepeaks will be separated clearly. See alsonominal mass.

Multiple reaction monitoring(MRM): A specific experiment on atriple-quadtupole mass spectrometer,in which a parent ion is filtered inthe first quadrupole Q1), a collisionis then induced between the parention and a molecule (usually a gas suchas argon) in the middle or "RF only"quadrupole (Q2), followed by detec-tion of a specific product ion from thatcollision (Q3). Used in high-through-put quantitative analyses in the phar-maceutical industry especially.

MSn: A term coined with the resur-gence of ion traps denoting the abilityto choose a specific ion present in theion source and fragment it; repeatingthe procedure to increase specificitywhen attempting to identify an analyte.The procedure can be repeated (numberof repeats = ) providing the chosenfragments have sufficient energy andenough sample and time are providedfor the experiment to continue.

Nominal masst Electron ioniza-tion MS often relies upon perfluo-rinared compounds such as perfluo-rotributylamine (nominal molecularmass of 671) to calibrate the m/z

scale. That is because the integermass of an ion is almost the sameas its monoisotopic mass. If an ionexceeds a nominal mass of 1000 Da,there is no observed nominal m/zvalue peak in the mass spectrum.The monoisotopic mass peak is offsetfrom where the nominal mass peakshould be observed by an amountequal to the mass defect of the ion.For single-charge ions with massesabove 500 Da, using techniqueslike electrospray with transmissionquadrupole or quadrupole ion trapmass spectrometers that have unitresolution throughout the m/z scale,the isotope peaks will be separatedclearly. See also monoisotopic mass.

Open access (OA): Also referredto as "Walk-up systems," these areworkflow controls allowing a fullytrained operator to create completeLC- or GC-MS methods and makethem available to a large number ofnonspecialist users, giving them accessto advanced technology without therequirement for extensive training.

Parent ion: More properly referredto as "precursor;" a generally inter-changeable term with "molecular ion."Use of this term infers the presence ofa product ion in an MRM scheme. SeeProduct ion.

Particle beam: Originally devel-oped at Georgia Tech and dubbedMAGIC (monodisperse aerosol gen-erating interface for chromatography)by Browner and colleagues. The tech-nique was later refined and is referredto generically as particle beam. TheLC stream is heated and nebulized toremove the solvent. Vacuum pumpsdraw the solvent vapor through skim-mer cones in series (usually two). Theresult is a "dried" particle that accel-erates through the momentum separa-tor and impacts the mass spectrometersource producing fragment ion spectrasimilar to traditional GC-MS. Thistechnique is now rarely used.

Probe: Also solids probe or directinsertion probe. A metal rod insertedinto the mass spectrometer sourcethrough a vacuum lock. Samples canbe applied to the tip of the probe andplaced into the path of an ionizingbeam. Typically used for EI and

1 4 2 LCGC NORTH AMERICA VOLUME 27 NUMBER I FEBRUARY 2009

Other single sample manual experi-ments. Samples also can be appliedin an ionization enhancing matrix asin the case of FAB.

Product ion: Formerly called"daughter ions," these are the result ofcontrolled experiments in which a pre-cursor (ot "parent") ion and moleculecollisions are induced to cause frag-mentation. The collision gives rise toa product ion specific to the precursorion and is used as a means of positiveidentification. See MRM.

Protonated molecular ion: Someforms of ionization produce ions by aproton transfer process that preservesand promotes the appearance of themolecular ion itself (the end resultreferred to as a pseudomolecular ion).In chemical ionization, for instance,the sample is exposed to an excessof reagent gas to form a protonatedmolecular ion (represented M+H).The reverse process can produce nega-tive ions. Transferring the proton tothe gas molecule can, in some cases.

S o m e t h i n g Better n i t n v,iri;>iiii.itls

J* DAISD CD.. LTD.http://www.dais09el.com / Email: sllica@daiso.co.jp

np/e isquest contscf

DAISO Fine Chem USA, INC.807 Aldo Ave. Suite 104. Santa Clara, CA 95054Tel: +1-408-855-8789 Fax: +1-408-855-8784E-mail : 5illca@dal50.us

produce the negative ion (MH) ordeprotonated ion.

Pseudomolecular ion: Usuallyrefers to the adduction of a proton (forexample, M+H) or ion (for example,M+NH^ derived from the ammoniumsalt commonly used in the mobilephase) that alters the analyte of interestin some (relatively) easily identifiablefashion. The charge allows manipula-tion by the mass spectrometer.

Quadrupole: The underlying fea-ture for the most prevalent type ofmass spectrometer. Four rods (often nomore than 1 in. in diameter and lessthan 12 in. long) are held parallel toeach other (about 1 in. apart) in twocollars. Filtering, or passing a givencharged particle along its length, isaccomplished by applying DC and RFvoltage to the rods. Different masses(with associated charge) are affected bychanging the RF-DC conditions. Therods are connected as paired opposites each set alternated as the positiveand negative poles by the RF source.

For a given calibrated setting, onlyparticles of corresponding w/z willpass (approximately equivalent tomolecular weight). The same settingwill cause higher weight particles tomiss the detector by passing in obliquefashion to the poles (the voltage set-tings having little or no effect) andlighter particles to become entrappedwithout reaching the exit and beingdetected. Quadrupoles can change andstabilize these mass filter field condi-tions quickly allowing more than onemolecular weight to he observed byscanning over time, although fewercharged particles are therefore detectedfor any given molecular weight.

Functionally, the single-quadrupolemass filter, used alone when matrixinterference is not an issue, can be joinedwith another to enhance discrimina-tion of a given analyte among manyin a background (from the matrix forinstance). Early designs used a thirdquadrupole-rypc device between thetwo as the collision cell (hence, the term"triple quadrupole"), while mote recentdesigns use specialized devices and arereferred to as "tandem quadrupoles."

Quadrupole time-of-flight (QTOF):This mass spectrometer couples a TOF

www.chromatographyonline.com FEBRUARY 2009 LCGC NORTH AMERICA VOLUME 17 NUMBER 2 1 4 3

instrument with a quadrupole instru-ment. This pairing results in the bestcombination of several performancecharacteristics: accurate mass measute-ment with a TOF instrument and[he ability to carry out fragmentationexperiments between the two along withhigh-quality quantitation and mass fil-tering. A QTOF instrument's high massaccuracy falls within a few parts per mil-lion of the ttue, calculated, monoisotopicvalue, and its high resolution as muchas 10 times higher than a quadrupoleinstrument's permits determinationof empirical formulas according to massdefect (where the critical mass value ofhydrogen and other atoms present serveas a differentiator).

Radical cation: Most stable organiccompounds have an even numberof total electrons because electronsoccupy atomic orbits in pairs. Whena single electron Is removed from amolecule, the total electron countbecomes an odd number, a radicalcation. The molecular ion in a massspectrum is always a radical cation (asseen in EI), but the fragment ions canbe even-electron cations or odd-elec-tron radical cations, depending uponthe neutral (uncharged) fragmentlost. The simplest and most commonfragmentations are bond cleavagesthat produce a neutral radical (oddnumber of electrons), and a cationhaving an even number of electrons.A less common fragmentation wherean even-electron neutral fragment islost produces an odd-electron radicalcation fragment.

Radio frequency: see "Direct current."Resolution (10% valley method):

The minimum separation between twoneighboring masses of approximatelyequal response for the mass spectrometerto distinguish between ions of differentmass-to-charge ratio. More typicallyused with magnetic sectors; equal to theaverage mass of the two particles dividedby the difference in their masses.

Resolution {A/AM): More com-monly used as a measure in which agiven mass is divided by the resolutionat full width half height maximum(FWHM). The 10% valley method wasprevalent with magnetic sector instru-ments and requires that the neighbor-

ing masses be of equal intensity. Forinstance, a typical resolution value fora quadrupole is 0.6 amu at FWHM.Measured using an acquired peak atm/z 3000 (equivalent to daltons or amu)equals a tesolution of 5000. The tesultsof the two techniques are roughlycomparable, with this method typi-cally yielding values double the valleymethod. Equal to the mass divided bythe peak width at 50% peak height.

Root-mean square (RMS) errormeasurement: A comprehensive methodof evaluating instrument mass accuracy

measurement capability, which resemblesintended use by calculating the RMSerror. The RMS error is calculated usingthe following relation, in which E sthe parts-per-million errot and is thenumber of masses considered:

Scanning: See also selected ion moni-toring, quadrupole and ion current. Con-trol voltages (DC and RF) are adjusted

I

FROM CONCEPTION TOFABRICATION, WE ENGINEERIDEAS INTO REALITY.

An independent, family-owned company,J.G. Finneran Associates designs,engineers and manufactures uniquesample handling produas. ''

In addition to glass and plastic vialmanufacturing, our capabilities includevacuum forming, decorating, liningand custom packaging.

Contact us today to receive a freesample pack and our product catalog.

J.G. Finneran. Driven to improveperformance in your lab.

Visit our website!Compare productson-line to find thebest fit forour application.

www.jgf inneran.com

144 LCCC NORTH AMERICA VOLUME 27 NUMBEH2 FEBRUARY 2009 www.chromatographyonline.com

DataApex

Clarity

D.I l.'i Apex

Clarity

LOOuD)2

S6uDroo o-

ProvenSOFTWARECHROMATOGRAPHY

easy to install and easy to use 170+controlled instruments

Request your DEMO VERSIONWWW.DATAAPEX.COM

by the computet over a given time toscan (detect) atiy chatged patticles in thespecified range. The benefit of being ableto detect mote than one species is at theexpense of sensitivity because some ofthe desired particles undoubtedly will beavailable to the detector while it is set todetect elsewhere in the range.

Selected ion monitoring (SIM): Alsocalled selected ion recording (SIR); refersalso to quadtupole and scanning. TheDC and RF voltage settings on the quad-rupoles can be adjusted to pass only onecharged particle (a single mass-to-chargeratio) through to the detector. The resultis a dramatic decrease in noise, allowingthe signal to appear as a dramatic increasein sensitivity (all particles ofthat m/z atebeing detected all the time) at the expenseof any other particles in the mixture beingdetected at all.

Thermospray: Although this type ofinterface has been in the literature forsome time, it was popularized in theearly 1980s. Vestal and Blakely shouldbe given credit fot creating the firsttrue commercially feasible interfacebetween LC and MS. LC solvctit at aflow rate of approximately 1 mL/minis heated in a probe (insulated tub-ing approximately 1-2 ft long and75-150 [xm internal diameter) andthe resulting vapor is sprayed into themass spectrometer. Ions created bythe desoivation of the aerosol dropletsinside the mass spectrometer enter theanalyzer (at right angles to the spray)and are affected by the lens voltages(see scanning, total ion current).

The spectra produced are termedsoft-ionization spectra because littlemeaningful fragmentation is produced.An intense molecular ion is producedand while the single ion can be of littleadvantage in some backgrounds andmixtures, it is advantageous for highmolecular weight confirmation at greatsensitivity and fot filtering a targetion for further fragmentation (MS-MS). Literature reports low picomolesensitivity for vitamin D metabolites.The interface was chosen generallyfor highly polar applications such asmetabolite work before the refinementof APCI in the early 1990s, and itworked poorly as organic content inthe liquid increased.

Time-of-flight (TOF) mass spec-trometer: A mass analyzer that separatesions of different mass-to-chatge ratios bytheir time of travel thtough a field-freevacuum tegion after having been giventhe same kinetic energy. The velocity ofthe ions is dependent upon their mass-to-charge ratio and as the ions are travel-ing ovet a fixed distance, the time takento reach the detector allows the mass-to-charge ratios to be determined, withheavier ions taking longer.

Tuning! Typically refers to optimiz-ing the interface lenses and flowinggases to achieve a desired response fora specific analyte under a set of operat-ing conditions as opposed to calibra-tion. Calibration defines the massacquisition and reporting function.Hardware settings of lenses and relatedcircuits in conjunction with creationof a software look-up table sets a stableinstrument s response, corrected to alist of known masses from a flowingstream of calibrant such as polyethyl-ene glycol (PEC) or NaCsl.

Vacuum (ton): Equivalent to 1 mmHg (1 psi = 51.7 torr = 0.069 baror atm). The analyzer portion of themass spectrometer typically must bemaintained at a minimum of 10"^ torrto allow discrete passage of the ion-ized particles. Pressures tending towardatmospheric cause ion-moleculat inter-actions that can ptoduce random resultsin the charged particles being detectedfurther downstream. Under controlledconditions, such collisions are inducedat low vacuum (higher pressure) fortechniques such as Cl.

Michael P. Balogh"MS The PracticalArt" Editor MichaelP. Balogh is principalscientist. MS technol-ogy development,at Waters Corp. (Mil-ford, Massachusetts):a former adjunctprofessorand visiting scier)tist at RogerWilliams University (Bristol, Rhode Island);cofounder and current president of the Soci-ety for Small Molecule Science (CoSMoS) anda member of LCGC's editorial advisory board.

For more information onthis topic, please visit

vvww.chromatographyonline.com

\