Topics in..
. Chemical lrutrumeetation
Edited b y 5.
Z. LEWIN,
N e w York University, New York 3,
N.Y.
These articles, most of which are lo be contributed by gzlest authors, are intended lo serve the readers of lhis JOURNAL by calling attention to new developments in the theory, design, or availability of chemical laboratory instrumenlation, oi by presenting useful insighls and ezplanations of topks that are of practical importance to those who use, or teach the use of, modern instrumentation and instrumental techniques.
XIII. Mass Spectrometry Stephen E. Wiberley ond David A. Aikens, Deportment o f Chemistry, Rensreloer Polytechnic Institute, Troy, New York I n an article (1)entitled "Mess Spectroscopy" in Chemical and Engineering News in November 1954, Dudenboatel and Priestly wrote "Don't throw up your hands when mass spectrometry is mentioned. I t is not a black art, i t is an understandable science." Mass spectrometry is an understandable aaienre to be sure hut even in 1964 in some respects i t is still tinged with a black magic whioh this article will endeavor to point out. The mrtss spectrometer has many valuable applications. I t can provide qualitative and quanbitative analysis of complex mixtures of gases, liquids and, less readily, solids-ither inorganic or organic. Until the advent of the gas chromatograph the mass spectrometer was virtually the only instrument suitable for the rapid anslysis of complex petroleum mixtures or process streams. Thereis little doubt that the gas chromat+ graph with its lower cost and relative simplicity has become a keen competitor. Still the mess spectrometer has such strong points as its extreme sensitivity and ability to identify unknown substances and to establish molecular structures. The mass spectrometer detecte the atomic arrangement within the molecule rather than any over-all properties of the molecule as a whole. Hence even such subtle structural features s s position isomerism have s strong influence and are readily revealed. Mass spectrometry can be as useful as the other spectroscopic techniques in establishing chemical structure and can often provide information that is unobtainable in any other manner. For example, octaborane ( 8 ) and boroxine ( 8 ) . ,were first identified ss chemical entlhes with the mass spectrometer. Before discussing the details of mass spectrometry let us examine in x general way a typical mass spectrometer and the pnnnples upon wh~ch . .r t depends. All
1 . .
maas spectrometers are very much alike functionnlly in that they perform three
basic tasks: the creation of ion fragmente from the asmple, sorting of these ions according to mrtss (strictly mass/ charge ratio), and measurement of the relative abundance of ion fragments of each mastss. This information presenbed graphically is generally known as a ma88 spectrum snd gives a detailed picture of the fragmentation of the sample molecule. Hence mass spectrometry is not merely useful for qualitative and quantitative identification, but is an extremely powerful tool for the ovaluetion of molecular structure. Mass spectrometers vary more or less according to the manner in which these three functions are performed. Techniques for creation and counting of ions are relatively standard with minor differences between instruments. The ion resolution system on the other hand is ususlly the most distinctive feature of a msss spectrometer and provides a very convenient basis for classifying instruments. A number of unique ion resolution systems have been devised hut they d l can he placed in either of two major cetegnries. The first and by far the moat widely used type of resolution system depends on deflection of a beam of ion fragments in magnetic or electric fields. Resolution by magnetic field deflection has become so popular that magnetic deflection instruments are commonly referred to sri "conventional" msss spectrometers. The second type of system achieves mass resolution from differences in velocities of ions of equal kinetic energy but different masses. Mass spectrometers of this type are generally known as "time of flight" instmments. At present deflection inst,rumenta are much more widespread and we will discuss them first and consider timeof flight instrumentslater. We can now examine the operetion of a typical commercial mass speetrometer in some detail. The steps necessary t o secure the mass spectrum are outlined in Figure 1. This diagram refers to a
S. E. Wiberley is Arso&te Deon o f the Graduate School of R.P.I. He xor educated a t Willioms College I0.A. 19411 m d R.P.I. 1M.S. 1 9 4 0 ; Ph.D. 19501. He teacher analytical chemistry and ir author of three lextbookr and opproiimotely SO papers in such research o r e m or spectroscopy and inrtrurnentd anolyris.
David A. Aiken. ir A I I ~ . , ~ " ,Profelm, of Chemistry a t R.P.I. He was educated al Northeortern University (0.5. 19541 and M.I.T. (Ph.D. 1960). He teacher onolytiml =hemirtry and has releorrh interests in m. ordination chemistry and eledrochemitry.
Consolidated Electrodynamics Corporation Model 21-l03C mass spectrometer hut applies with only minor variation to most magnetio deflection instruments. (a) The ssmple may be either a gas or 8 liquid. For liquid samples wit,h vapor pressure helow 20 micrnns a t room temperature both the liquid sample and the inlet syst,em are heated. ( b ) The ssmple is introduced to the evacuated inlet system and expanded to x volume of 3 liters st a, final pressure hetween 1 and 100 microns. An nrcurate pressure measurement is made with
(Conlinmd o n page A76)
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MICOMIHOMEIER #RID61 UMPUNfR
RC t ORON l E OSCILLOERAPH
9
Figure 1. Diagram of o typical magnetic deflection moss =pectrometer. Electrodynamics Corporation).
the Micromanometer, a. device designed for absolute pressure measurement. in the micron range. ( e ) The gas nmlecules pass through a pinhole restriction known as a leak and enter the analyzer assembly which is maintained nt a pressure below mm Hg. ( d ) The gas molecules are bombarded by electrons from a heated filament. A few collisions merely remove an electron from the gas molecules producing positive molecular ions. Many more collisions break one or more chemical bonds as well causing s series of positive ion fragments. ( e l The ions leave the ionization region thmugh collimating slits and are srcelerated by an intense electric field. ( f ) The rapidly moving ions are diverted into circular paths by a magnetic field parellel to the slits and perpendicular to the ion beam. Each ion follows s circular path of radius r which is proportional to the mass of the ion according to earlation
where r is the radius of curvature in cm, H is the magnetic field strength in gauss, m is the mass in grams, e is the charge in sbcoulombs of the ion, and V is the accelerating potential in abvolts. g Thus sorted by mass the ions traverse an arc of 180" where ions of the rtppropriitt,e mass pass through resolving slits and strike a collector giving up their charges whieh are amplified electronically. ( h ) The amplified ion currents are fed to a recording oseillograph containing several light beam galvanometers each of different sensitivity. A beam of light from each galvanometer mirror is projected ont,o s moving roll of photographir paper. (i) Positive ion beams of increasing mass are brought successively to focus s t the rollector by continuously decreasing the accelerating voltage. As each heam sweeps across the collector its intensity is recorded on the paper giving
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ICourtery Conwlidoted
a oontinuoua plot of intensities of successive ion beams. The peak height is proportional to the number of positive ions of a given mass and the ion mass is indicated on the abscissa.
tron-molecule collisions. The ionization potential of methane is 13.1 electron volts and a t least this amount of energy must be transferred to a methane molecule in a collision to create a positive ion. If the electron energy just equals the ionization potential the electron must transfer all its energy to the molecule to cause ionization. Furthermore no kinetic energy can be added to the resultant molecular ion during the collision. T h i ~ combination of events is extremely unlikely. Hence only a few molecular ions are formed, causing a weak peak a t mass 16. This peak, corresponding to the molecular weight of the sample molecule, is known as the parent peak. As the ionizing electron energy is increased above 13.1 electron volts the probability of ionization on electron impact increases and the mass 16 parent peak becomes more intense. The minimum ionizing voltage necessary to form a given positive ion is known as the appearance potentid of that ion and the resultant plot of pesk intensity versvs ioniring voltage shown in Figure 2 is known as the ionization efficiency curve
Factors Influencing Moss Spectra
We have now established in a general way the over-all process by which mass spectra are produced. Now we must consider this process in detail. The operations that lead from sample molecules to rounting of ion fragments of each mass are influenced both by the molecular structure of the aample and by the experimental conditions under whieh the ions are produced. Hence mass spectra sre influenced strongly by instrumental varhbles and to employ mass spectrometry to full advantage it is necessary to consider a t a molecular level the factors influencing the relative abundance8 of ion fragments in a massspeotrum. The structure of the sample molecule exerts the strongest influence on the mass spectrum and an ultimate goal is the relation of molecular structure to mess spectra. However the influence of moleonlar structure on mass spectra is more easily understood once the process of ion production is established. Hence we shall consider first the production of ions from simple molecules. ( a ) Ionizing Voltage. If methane molecules are bombarded by electrons with energies of only a few electron volts no positive inns are produced by elecTable I.
The value of appearance potentials aa a means of establishing ionization potentials is clear hut unfortu~atelyappearance potentials are seldom as well defined as we would like. Theoretically, ionization should commence st a discrete value of ionizing voltage and above this value of ionizing voltage the pesk intensity should increase linearly with ionizing voltage, ultimately leveling off a t a n ionizing voltage of from 30 t o 50 v. Experimental ionization efficiency curves are linear only when the ionizing voltage exceeds the appearance potential by a t least 1-2 v. At ionizing voltages immediately above the onset of ionization, however, the ioniatttion efficiency curve shows distinct curvature. This curvature, which obscures the value of the
Influence of Ionizing Voltage on Moss Spectrum of Methane
--
Mass/ charge
Figure 2. Ionization efficiency curve of methane showing appearance potential of CHI +.
Particle
12 C' CH + 13 CH%+ 14 CH, 15 CH4+ 16 Total ionization +
Ionizing Voltage 7-70 Volta-% Total % Total ionimtmn Intensity ioniaation
15 Volt.--
Intensity ...
1 360 10,250 15,500 26.100
Ionizing voltages are nominal values. Spectra secured on CEC Model 21-260.
0:&4 1.7 39.2 59.4 . ..
960 2,850 5,940 38,400 46,200 94.400
(Continued o n page A78)
1.0 3.1 6.3 40.6 4R.8
appearance potential, ia attributed principally to the energy spread of approximately two electron volts in the ionking electron beam. A few electrons always have energies significantly above the mean electron energy and hence ionization appears to commence below t,he ionization potential. This premature onset of ionization esn be eliminated with a monoenergetic electron beam; then the ionization efficiency curve becomes essentially linear and the appearance potential is well defined. If the electron energy ia increased sufficiently above the ionizat,ion potential, the excess energy transferred to the molecular ion exceeds the value required to breek a bond within the molecule. The molecular ion breaks apart s t that hond producing an ion fragment and a neudral fragment. The onset of fragmentation corresponds to the appearant* potential of the fregment peak. As before, the probability of fragment produetion increases with increasing electron energy and s plot of fregment peak intenaity vemm ionizing voltage is similar to the ionization efficiency curve. Further inereaae in electron energy leads to more severe fragmentation in which several bonds are ruptured. Hence a n increase in ionizing voltage enhances the relative intensitiee of the low mass peaks a t the expense of the high ma88 peaks and the spectrum is extremely sensitive ta changes in ionizing voltage. The influence of ionizing voltage on fragmentation of methane is shown in Table I which lists peak intensities in the mass spectrum of methane secured a t ionixing voltages of 15 and 70 v. Clearly the total production of ions is higher a t the higher ionizing voltage and the distribution of peak intensities follows the expected trend. Thus increasing the ionizing voltage from 15 to 70 v enhances the absolute intensity of the mass 13 peak 2800-fold. The absolute intenaity of the mass 14 peak increases by a factor of I7 while the absolute intensity of the parent peak a t mass 16 increase8 by only a factor of 3. The net effect of higher over-all intensity and more severe fragmentation caused by increased ionizing voltage is to increase the relative intensity of the fragment peaks at, the expense of the parent peak. At 15 v ionizing voltage the parent peak represents 59.4% of the total ionization; a t 70 v ionizing voltage the parent peak represents only 48.8% of the total ionination. The maas 13 peak and the mass 14 peak were resper? tively 0.004 and 1.7';; of the botal ionirst,ion s t 15 v ionizing voltage; s t 70 v ioniring voltage the mass 13 peak and the mass 14 peek contribute 3.1 and G.3yGof the total ionization. Although the peak intensities are ext,remely sensitive to ionizing voltwe a t low value ionizing voltage, the relative peak intensities beronle fairly ronst,ant and independent of ioniring voltage once the ionizing voltage exceeds 50 v. Therefore most ma88 spectra are recorded a t an ionizing voltage of 70 v to ensure reproducibility. Sometimes doubly rhnrged ions appear
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in mass spectra. As a n example the mass spectrum of carbon monoxide contain8 a mass 14 peak due to C o t + in addition t,n peaks a t mass 12, mass 16, and mass 28 that correspond respedvely to C+, 0 and COt. Ihuhly charged ion peaks are usually of low intensity, generally of the order of 1% of the most intense singly charged ion peak. Ihubly charged ions of odd mass are especially easy tr, recognize, of course, because they appear s t half integral mms. For example the peak a t mass 39.5 in the pyridine spectrum arises from the douhly ch:wged pyridine moleruler ion, CEH,N++. Lhubly charged ions are indicative of high T electron density in the molecule. Therefore doubly charged ions are characteristic of unsaturated, aromatic and heterncyclic molecules and are often valuable for t,he qualitative identification of such compounds. ( b ) Molecular S6~ueha~e.Mass spectmmetry is a powerful technique for establishment of molecular st,ructure, because the molerular structure of the sample exerts a profound influence on the mass spectrum. However interpretation of mass spectra is often obscured because mass spectra are highly complex even for relatively simple moleeules. For example the mass spectrum of nbutane contains 37 peaks and that of etlranol contains 21 peaks. The mass spectra of larger molecules are correspondingly more complex and u s u ~ l l y contain peaks corresponding to virtually every maas particle that e m he constructed from the atoms of the sample. However peak intensities vary over a range of 10,000 to 1 and relative peak intensities are extremely sensitive to the molecular strurture of t,lre sample. Thus peak intensities ronvey most of the iniormat,ion in n mass spectrum and must he considered in establishing the molecular structure of the sample. Because mass spectra are inherently complex their interpretation is always empirical. Two distinct approaches to interpretation of mass spert,r.z have developed and will he considered. The first approach is the simpler of the two hut ie limited severely in application. This approach attempts to define the influence of s. single struotural feature or functional group on mass spectra. Spectra of an appropriate series of compounds are examined to determine common trends and a series of rules is established that define the most intense peaks. This approach resembles the use of infrared correlation charts to identify funct,ional groups fmm their eharacteristic absorption bands. As an example mass spectra of primary alrohols are characterieed by an intense peak a t mass 31 artrising from the fragment +CH>OH. This approach is attractive because of its simplicity and rules such as thitr are useful for interpreting spectra of simple molecules. However this approach often fails when applied to molecules containingmore than one functional group. The failure of the simple functional group approach arises from interactions among functional groups. Contrihutkm of functional groups to a spectrum are
(Continued m page A821
often not additive. A successful general %pproach to interpretation of mass apectra must take account of the influence of the entire muleeule on the mass spectrum. Each portion of the molecule doea not influence the mass spectrum independently. Rather the entire molecule is first ionized and the resulting positive ion undergoes fragmentation. Fragmentation is basically a set of competitive parallel reactions that start with a common reactant and produce unique products. The distribution of these products is determined by the probability of each reaction and eaeh reaction probability depends on the net effect of all the structural features of the positive ion. Thus interpretation of mass spectra must take weaunt of the inhence of the overall #structure of the parent molecule on processes where chemical bonds are broken (and less often formed). This approach to mess spectrometry should have a familiar ring. I t is the same mechanistic approach that organic chemists have applied so successfully to the study of organic reactions under the general term "physical organic chemistry." The essence of the meehanistic approach is that the yield and the structure of the product(s) of a resetion are related to the structure of the reectctnt. Given the reactant structure, the reaction products can be predicted with reasonable confidence by consideration of s small number of basic principles. The same principles apply to mass spectrometry, except that now the products (ion fragments) are known and the problem is to establish
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the structure of the reactant (sample molecule). Thus physical organic chemistry provides a natural basis for discussion of mass spectral interpretation and utilizes a large body of organized information already available to organic chemists. This promising approach to mass spectrometry has been established principally by the work of McLafferty and of Biemann. In addition to material in numerous research papers, excellent summeries of both Biemam's (4) and McLafferty's (6)work m e available. The mechanistic approach to mass spectral interpretation is far too broad for complete treatment here. However a few examples of its application should give ample evidence of its potential. Because it employs besic chemical principles to interpret mass spectra the mechanistic approach should became iocreasingly valuable and more widely used. The mecbanirltic approach considers fragmentation processes as iduenced by two dominant factors: availability of an energetically favorable path and stabilization of the resulting fragments, especially, positively charged fragments. Thus extensive use in made of such familiar cancepta as bond strengths, osrbonium ion stabilities, bond migration, and the principle that reactions which require a drastic reorganization of the sample molecule are unlikely. In applying these concepts to mass spectra we must r e member that ion fragments of mass spectrometry are in excited states. However our concepts of structure and stability are derived from group state
particleb and are applied in mass spectrometry only because we know nothing of these properties in excited particles. Therefore a t present predictions are educated guesses although usually of considerable accuracy. The following example taken from Biemam's work (4) illustrates this s p proaeh to msss spectral interpretation. Mass spectra of alcohols, mercaptans, and amines are characterized by intense peaks that arise from cleavage of the carbon-carbon hond adjacent to the heteroatom. The effectiveness of the heteroatom in stabilizing the positive charge is established as N > S >> 0 from spectra of bifunctional molecules such as a m i n o e t h o l containing two heteroatoms. The msss spectrum of aminoethanol contains two peaks arising from cleavage of the carbon-carbon hond and the intensity of eaeh peak ia a direct measure of the ability of the associated fragment to stabilize the positive charge. The +CH-NH, ~ e a ka t mass 30 contributes 57.0% -0; the total peak intensity and the + C H A H peak at msea I 31 contributes only 5.1% of the total ' pesk intensity. Hence the nitrogen frag- I ment stabilizes the positive charge 8Pproximately eleven times as effectively as does the oxygen fragment. Oxygen can compete somewhat more effectively with sulfur for the positive charge however. In the mass spectrum of thioethanol the +C&--OH fragment a t mass 31 represents 8.5% of the total ionization,
i
(Continued on page A84)
I
as conlpsred to the +CHz-SH fragment s t mass 47 which represents 13.7% of the totel ionization. On a relative hasis the peak of the sulfur fragment is approximately 1607, as intense as the peak of the oxygen-containing fragment. Both peaks are of lower intensity than the corresponding peaks of aminoethanol because the spectrum of thioethanol contains many more peaks. Thus on a relative basis oxygen is the least effective heteroatom i n stabilizing the positive charge, sulfur is approximately 60L7, more effective and nitrogen is approximately 1000'%omoreeffective. The stability of an ion fragment is strongly enhanced if the carbon backbone can help stabilize the positive charge; that is, if the carhorium ion corresponding to the carbon chain has a high stability. Carbonium ion stability increases with substitution and ion fragments containing a highly substituted carbon adjacent t,o the heteroatom are highly favored. The effect of alkyl groups can even invert the order of peak intensities from that predicted from heterontoms. Thus in the mass spectrum of I-amino-Zmethylpropitnal-2 the (CH8)2-C--OH peak a t mass 59 is in faot more intense than the +CH,NH* peak s t mass 30. On the other hand the carbon chain e m also reinforce the influence of the hetero~tom with the greater chsrge stabilizing ability. Then the fragment containing this heteroatom is doubly favored and the corresponding peak becomes very intense. An example of this situation is found in the spectrum of 2-methyl-2-aminopropanol-I, in which the hydroxyl and amino groups of the previous example have been interchanged. The 58 peak corresponding to the fragment (cH~)~-&-NH~ is fifteenfold more intense than the 31 peak corresponding to the fragment +CH?-OH. I n a similar manner m m a t i c rings cen compete very well for the positive charge and often virtually suppress the "characteristic" peaks of functional groups located on side chains. For example an intense peak s t mass 31, arising from the fragment +CH2-OH, is eherseteristie of primary alcohols. The 31 peak is in fact the most intense peak in the ethanol spectrum but in the spectrum of 8phenylethanol the 31 peak is only onefifteenth as inten= as the 91 peak corresponding to the aromatic fragment r'-,--, .H.+ . Just as the intensity of each fragment peak depends on the molecular structure of the sample, SO a180 does the intensity of the parent peak. The parent peak is of special interest because it permits direct determination of molecular weight. Often, however, the parent peak is of extremely low intensity and i t may even escape detection. Furthermore the parent peak is not $ways the peak of highest mass. Consequently moleculer weight determination by mass spectrometry is sometimes a difficult task. Molecular ions with electronegative heteroatoms can abstract s hydrogen atom from a neutral molecule and cause n "p 1" peak one mass unit above the parent peak. Such " p 1" peaks often
+
+
(Continued on page A88)
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ocerlr in t,he spectra of esters, alcohols, amines, and nitriles and can he used for q u a l i t h v e identification of these compounds. Reemse the " p 1" pesk arises from a. himolecular collision in the ionizing chamber, the intensity uf n "p 1" peak is proportional to the square of the sample pressure. This unusuwl 1,ressore-dependence distinguishes "p 1" penks from all other peaks in t,he mrss spectrum that arise from intramolerular pmcesses and which vary linearly with the sample pressure. The intensity of the parent pesk is enhanred by the presence in the sample molerole uf a elect,ron systems from which a n electnm e m be removed rather easily. However if the molecular struct,ure contains features that stabilize fragments strongly, t,hen fragmentation pmeesses are favored a t t,he expense of the intensity of the parent pesk. Aromatics ;md conjugilt,rd olefins exhibit intense parent peaks; t,he parent pesk of naphthalene is 45% of the total peak intensity. On the other hand highly branched hydrorarhons undergo extensive frsgmentation and show very weak parent peaks; t,he parent peak of 3,3,5-trimethylheptane represent,a unly 0.00770 of the t,ot,al peak int,msity. (c) A f o l ~ e ~ ~ lRearrangement. ar Sometimes penks nppeer in mass spectra s t rnrtsses which do not correspond t o any group in the molecule. For example a 29 peak corresponding t o CIHii orcun in the snccbrum of isonentane and no fragment of the parent molecule aorresponds t o this peak which arises from :L moleculnr rearrangement. Such mnlw-
+
+
+
.
~
~~
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u l ~ r renrrmgements arc common in mnss spectra and are often associated with fnirly intense peaks. For example, the 2!1 pe:k in the neopentrme spectrum is the third most intense peek and runtributes l i % nf the toal peak intensity. I\lol,dferty (6) has postulated thxt rearrmgement peaks arise irum intmmdecol;~r hydrogen migmtirln. Heme the appe:mnre of renrnmgenient pr:tks in a mass spectrum plnees detinite strurt u r d restrictions on the piwent rnolpeule. A good example of this npprnnrlr to mr~lceulnrrearrangements is found in Mc1.sKert.v'~ (6) study of :~Ikyl ketones. Alkyl ketones with Cmr side rhnin nt least three rnrhon atoms long ur,derga rearrnngement through a cyclic 6 nlembered transition state with the protons of the r c:rrhon adjacent to the carbon,yl oxygen. A proton on tho r carbon migrates t o the cnrbonyl oxygen and t,he fl and r amhons are eliminated ss a neutral olefin moleaule. The protonsted cmbonyl fragment rptrains the positive charge and is detectrd in t,he ma88 ~ p c c t n l m . ( d ) Isotope Effects. Until this print we h;bve considered t h a t nn ion irngment of given chemical structure rontribut,os to m l y one peak in the mass sper:tronl. Often t,his is not true. Many elements including carbon are made u p of more than one isotope, and fragments contzaining t,hese elements will contribute to mrm than one mass peak. The isotopic abundance of esrbon is 98.8UY0Cl2 nnd I .C . Hence the mass spectrum uf metlrsnc wnt:&ins a peak a t mass 17 t,hnt is npprtmirn:rtrly 1.17; as int,enue ;as thr peak a t nmss 1 6 . I n propane tlre:e is
s l.lCirchance that each carbon will be C13 and thus a peak orcurs s t mass 45 that is 3.B1i; as intense as the normsl parent peak a t mass 41. The magnitudes of such isotopic peaks one mass unit shove the normsl parent peak arising from only the C1?isotope ran thus yield ronsidernble information shout the numher of esrbon toms in the mr,leaule. I n s similar nlsnner a fragment containing a single hrondno sturn gives rise to two peaks of nearly equal int,ensity separated hy two mass units, and t,his clmrneteristie pattern is virtunl indiratirm of a bromine atom in the parent nu,lecule. ( Filan~rnl ?ompoxition. The eomposit,iun r l f the filament in the ionization chamber often has a pdlfountl influenceon t,he mass spectrum of a given compound. Filaments are operated a t incandescent heat and reactions between the filament and s:mple gas are common. For cxample a new tungsten filament catalyses tlre breakdown of hydnwsrtwns into r a r b m and h.vdrogen. The carbon dissolves in the tungsten t o form n tunmten rsrbide surface which does not rntalyae the hreakdown of hydn,csrbmm Henre when n new t,ungsten filament is installed in n mnss sp~etronleter, fragmentation patterns are irreproducihle until soffirient hydroc;~rhm material has deromposed on t,he filament to rrmt the filament with tungsten rsrhide. Formation nf the tunwten carbide mating is achieved hy admitting n-butane tr, the ionizntion chamber for several Imurs. An interesting paper by Le Blme ( 7 ) is prrtinmt tcn this disrussirm lie (('ontinnrrl o n paor A911
Chemical Instrumentation Blanc showed that aeetd gives different mass spectra on an uncsrhonked tungsten filament and a carbonized tungsten filament. Apparently all gemdiethen wit,h a ,%hydrogen atom decompose on the bare tungsten filament into alcohols and vinyl ethers. Far reliable mass spectrometric analysis of these types of compounds n eerhonized filament must be used. The extent of carbonization of n filament can be checked by measuring the spectrum of a gemdiether such as acetal. The inconvenience of tungsten filaments has led to use of other metals for filaments. Rhenium filaments, although not as rugged as tungsten filaments, give reproducible spectral pattern8 without coating and are widely used. (I) Source Temperatwe. Fragmentation patterns of complex molecules are extremely sensitive to the temperature of the ion source. Even a, small increase in source temperature strongly increases the extent of fragmentation. For example, the parent peak of 2,2,3-trimethylpentme is five times as large a t 175 as a t 225°C. Hence to emure .table frsgmentxtion patterns i t is necessary to stabilize the ion souree temperature to within a few tenths of a. degree, and commercial mass spectrometers have automatic controllers to regulate the source temperature. Furthermore, only spectra secured a t the same temperabure can be compared so i t is important to control the temperature a t which spectra are obtained. (g) Dependence of Spectra on Type qf fnslrumenl. I t would seem that careful attention to the instrumental variables such aa ionizing voltage and source temperature would ensure identical spectra from m y two imtruments. Unfortunately however, mass spectrs depend somewhat on the type of instrument from which they originate. Thus the spectrum of a compound seeured with a 180" deflection instmment ia different from the spectrum of the same compound secured on a. eycloidal focusing instrument, even when such facton as the source temperatures and ioniaing voltages are identical in each instrument. This effect is known as mass discrimination and each type of mass spectrometer exhibits mass discrimination to varying degrees. Even though each instrument produces the same initial distribution of fragments from the sample, the relative number of fragments of a. given mass counted s t the eolleetor will differ for each type of instrument. Masa spectrometers generally discriminate against ions of higher mass, with each type of instrument having its own relative response s t any mess. Because mass discrimination depends on the type of instrument, spectrs cannot be interchanged between instruments of different types, but me significant only in eomparison to spectra secured with the same t,ype of instrument. ( P a d Two, onelusion of "MassSpeelvometry." will appear i n the March ismre.)
