V o L 12, N o . 6, 1958 167

Comparisons of Spectra from Different Types of Mass Spectrometers*

V. J. Caldecourt Spectroscopy Laboratory, The Dow Chemical Company, Midland, Michigan

Abstract

A comparison of the cracking patterns of various compounds taken on different types of mass spectrometers shows large differences. Mass d~scrlmmatlon ~s a large factor, and this Is to be expected, par- ticularly when comparing 180 ° with 60 ° or 90 ° instruments. Vana- tmns because of the nature of the compounds are also ewdent.

Introduction

P u b h c a t i o n of mass spectra by the m a n y users of mass spec t rometers should aid in the f u r t h e r appl ica t ion of this me thod of analysis. Howeve r , the wide var ia t ions in c rack- ing pa t t e rns given by a c o m p o u n d on different types of mass spec t rometers have caused m a n y to ques t ion the value of publ i sh ing mass spectra. I t is the purpose of this paoer to compare c r ack ing pa t t e rns f rom three types of mass spec t rometers so t h a t the m a g n i t u d e of the var ia t ions can be seen and the s ignif icant variables possibly dis- covered.

Since there are no 1deal c r ack ing pa t t e rns available, comparisons m u s t be made on a relat ive basis. Spectra f r o m three sources are used: American Pefroleum Instifufe spec- t ra t aken on 180 ° Consolldaled Eleclrodynamics Corpora- f,on mass spect rometers , spectra f r o m the Texas Division of The Dow Chemical Company t aken on 60 ° General Electrw mass spect rometers , and spectra f r o m the Mtdland Divt~ton of The Dow Chemwal Company t aken on 90 ° mass spectrometers . In spite of the large n u m b e r of spectra in our files, re la t ively few spectra were avadable f r o m all three types of i n s t rumen t s . O n l y in the case of the ahpha t l c hydroca rbons are the same compounds used in all three comparisons.

Hydrocarbon Spectra

The graphs present the relat ive c r ack ing pa t t e rns as a f u n c t i o n of mass. T h e curves are p lo t ted f r o m points obta ined by t ak ing the rat ios of the c r ack ing p a t t e r n figures at several s ignif icant mass peaks a f t e r the pa t t e rns were n o r m a h z e d at mass 29 so as to provide a c o m m o n poin t of reference. Mass 29 was chosen as a po in t of refer - ence because it was as low a mass peak as could be used wh ich was of considerable size and for w h i c h data were avadable in near ly all cases. Some of the spectra of h igher molecular we igh t compounds had no t been scanned below mass 24. In a few cases ex t rapola t ions were necessary to ad jus t the posi t ion of the re la t ive c r ack ing p a t t e r n curves so t h a t they would be u n i t y a t mass 29.

I n t e r p r e t a t i o n of the resu l t ing curves can be made as follows. The relat ive c r ack ing p a t t e r n curves g iven in F igure 1 show t h a t at mass 15, the 180 ° mass spec t romete r has several t imes the response of the 90 ° spec t romete r and m u c h less response at mass 100. The o rd ina te is the f ac to r by wh ich one m u s t m u l t i p l y the re la t ive abundance of the ions at a g iven mass in the c r ack ing p a t t e r n of a 90 ° spec t romete r to ob ta in the va lue f o u n d in the c r ack ing pat- t e rn of a 180 ° spect rometer . Smooth curves were d r a w n to show t rends in the cases where mos t of the points ap- peared to fo l low a t rend .

The first f igure shows the results for some s t r a igh t chain al iphat ic compounds on the three types of mass

*Presented at the Sixth Annual Meeting, A S T M Commit tee E-14 on Mass Spectrometry, June 1958, New Orleans, La.

I0

.5

~ S

.5

i A ;TA fs

• ANE

n-DODEGANE

ALIPHA~CS 6 0 / 9 0 °

CYCLOHEXANE

n-OCTADECANE

/ n-DODECANE

= ~ , ~ * * / n - OC TA N E

• = ~ J = ~ G A N E

n-DO OECANE

0 4 0 80 120 160 200 24.0 FIG. 1. RELATIVE RESPONSE RATIOS OF PAIRS OF SPEC-

TROMETERS FOR SOME ALIPHATIC HYDROCARBONS

168

spectrometers. The three graphs are provided so peculiar- ities due to just one of the instruments might be detected.

The graph of the relative patterns of the aliphatics for the 1 8 0 ° / 9 0 ° instruments shows that the low mass peaks are of much higher intensity in the 180 ° patterns. This is undoubtedly caused by mass discrimination (1) in the 90 ° sector-type instrument. The 90 ° instruments were adjusted for maximum peak height in the mass 100 region. Since the accelerating voltage and the magnetic field m the ion source are not varied when scanning a spectrum, the mass discrimination effect is large at low masses and is observed for all types of compounds. Consequently, it could be corrected for when comparing spectra.

In addition to the mass discrlmmat~on evident at low masses, the relative cracking patterns of the different com- pounds show considerable divergence at the higher masses and even variations in the shape of the curves. This phenomenon is a function of the type of compound and therefore not easily predictable. The effect appears in all three comparisons, i.e. 180°/90 °, 6 0 ° / 9 0 °, and 180°/60 °.

Figure 2 shows a similar comparison for some substi- tuted aromatic compounds. Again, the relative cracking patterns show variations depending on the compound. As before, the phenomenon is common to all three types of mass spectrometers and is greatest in the comparison of the 180°/90 ° spectrometers. Some thought on the most outstanding differences in the operating parameters of these two types of mass spectrometers should give a clue as to the cause of the effect.

Compared with the 180 ° spectrometer, the 90 ° spec- trometer operates with about one-tenth as intense a mag- netic field in the ion source region, about one-tenth or less of the ion repeller voltage, and nearly one-hundredth the ion energy in the ion collector region. The low ion energy in the collector region occurs because the 90 ° spectro- meters are operated so as to suppress metastable ions (21. The ion accelerating voltages used in the 180 ° spectro- meters vary between 400 and 3800 volts, whereas the accelerating voltage for the 90 ° instruments is normally 450 volts. The ion source operating temperatures are near- ly the same, i.e. within ± 1 0 ° C . The most significant differences appear to be in the electric and magnetic fields in the source region and in the ion transit times. Some in- vestigations into these variables are shown in the next three figures.

Figure 3 shows how the response curves are modified when the repeller voltage is changed on the 90 ° spectro- meter. Increasing the repeller potential from 2 volts to 10 volts favors the high mass peak from the 90 ° mass spec- trometer and the high mass end of the curve is bent down- ward. Then the trend of the n-octadecane curve is more nearly like the trend for most of the other aliphatic com- pounds in that it levels out at the higher masses. This effect does not appear to be due to a change m the energy of the ionizing electrons because increasing the electron energy by 5 v results in a change in cracking patterns which is less than an order of magnitude of those shown in Figure 3. Similarly, the effect of repeller potential variation on the response of a 180 ° spectrometer can be seen in the two curves given for n-dodecane. The curve for n-dode- cane at the higher magnet current setting on the 180 ° mass spectromter is higher relative to the 90 ° spectrometer at the high mass end than the curve for the lower magnet current. On the 180 ° spectrometer, the same ions scanned at the higher magnet current wdl be subject to a higher repeller potential, and the higher repeller potential in- creases the size of the parent peak just as on the 90 ° mass

APPLIED SPECTROSCOPY

I0

.5

05

5

A ROMA TIC 1 8 0 / 9 0 •

NE UMENE

PROPYL BENZENE

AROMATICS 6 0 / 9 0 •

• 5 ~ CUMENE

BENZENE

AROMA TICS 1 8 0 / 6 0 "

5

I - ~ BENZENE \ ~ m-DI ETHYL BENZENE

I 5 TR, ETHYL BENZENE

I I I I I I I I I I I I I

0 40 BO 120 160 200 240 Mass

FIG. 2. RELATIVE RESPONSE RATIOS OF PAIRS OF SPEC- TROMETERS FOR SOME AROMATIC HYDROCARBONS

VOL. 12, No . 6, 1958 169

10

/ON DRAWOUT I

5 II / a o / 9 o *

,~ ~ '~, n-OCTADECAN E

i - ~ 2 v lOP

. I - ~ 0 . 9 A

I I I I I I I I I I I i I

0 40 80 120 160 200 240 ~IG. 3. RELATIVE RESPONSE RATIO OF 180 ° TO 90 ° SPEC-

TROMETERS FOR DIFFERENT REPELLER POTENTIALS

spectrometer. Actual ly the two experiments are not direct- ly comparable since a change m the magnetic field in the ion source region was involved, but ~t does show that the cracking patterns can be altered by these ion source variables.

The separation of the relative cracking patterns ac- cording to compound is not just a peculiarity obtained by comparing two types of mass spectrometers. Figure 4 shows the effect of a change in repeller potential on the relative patterns of n-dodecane and n-octadecane for a 90 ° mass spectrometer. Curve separation occurs as in the previous comparisons.

To investigate the effect of a change m ion transit time without disturbing the ion source, the potential of the ion collector on a 90 ° mass spectrometer was changed from zero to -43 volts. Our 90 ° spectrometers are norm- ally operated with the ion source box at zero potential, the analyzer at the accelerating voltage, and the ion col- lector at zero volts. Thus the ions are collected with the energy gained from the repeller potential which is about 2 electron volts. By making the collector 43 volts negative,

I0

5

I

/ON DR,,fWOUT I O v / 2 v

~ e ~ . ~ . ~ N n- DODECANE ~E

5 . 0 1 I t i I I I I I I I I I I

0 40 80 120 160 200 240

FIG. 4. RELATIVE RESPONSE OF A 90 ° SPECTROMETER FOR 10 VOLTS AND 2 VOLTS REPELLER POTENTIALS

5 \

/ON ENERGY 4 5 v / 2 v

~o

3 1 - ~ n-DODEGANE / .~ n - O C T A D E G A N E "~.5 %

. 0 J I I I I I I I I 1 i i I i

0 40 80 120 160 200 240

FiG. 5. RELATIVE RESPONSE RATIOS OF A 90 ° SPECTROM- ETER FOR IONS COLLECTED AT 45 VOLTS TO THOSE

COLLECTED AT 2 VOLTS

the ion energies were changed from 2 electron volts to 45 electron volts when reaching the 1on collector. Again a change in the cracking patterns is observed (Figure 5), and the variation ~s a function of the part icular compound used. The change in the spectra is beheved to be caused by a loss of the lower mass and higher mass ions by decom- position when collected at the lower energy. When the ions travel from the exit sht to the collector with 2 electron volts of energy the transit time in th~s region becomes a significant percentage of the total transit time of the ion thru the mass spectrometer tube. The departure of the curves from uni ty is small, mdmating a low rate of ion decomposition, but apparently an appreciable number of the ions are decomposing even after traversing the an- alyzer section of the mass spectrometer. Another possible explanation for this change in cracking patterns m~ght be that discrimination is taking place because of the initial energies of the ions. I t is doubtful whether this effect is significant since the collector is m a region of weak mag- netic field and there are no narrow slits revolved.

Spectra of Oxygena ted and Halogenated Compounds

The remaining graphs provide information on the effect of two strongly electronegative atoms on the rela- tive cracking patterns. Figure 6 presents the relative crack- mg patterns for several oxygenated compounds. Note that the same compounds were not used m all three graphs and that the oxygenated compounds give curves with somewhat greater variations in shape than do the hydro- carbons.

The concept of electronegativlty does not appear to provide an explanation for the very large variations in relative cracking patterns observed for the halogenated compounds shown in Figures 7. In these graphs the irregu- larities are so pronounced that smooth curves were not drawn. Chlorine and bromine have exceptionally large atomic dimensions and possibly this is a contr ibut ing fac- tor. The large variations m the relatwe cracking patterns for the different mass spectrometers probably indicate that the cracking patterns for these compounds on one type of instrument can be expected to be quite sensitive to the operating conditions.

Theory The separation of the relative cracking pat tern curves

as a function of the nature of the compound rather than as merely a function of mass shows that there is no simple relation between the mass spectra from the different spec- trometers. I t appears that none of the mass spectra can

170 APPLIED SPECTROSCOPY

5

O X Y G E N A T E D

1 8 0 / 9 0 °

I- B U T A N O L I

t \ \ • 5 ~ i ~ n - H E X A D E C A N O L

O X Y G E N A TED 6 0 / 9 0 o

AOETAL-- ,ONOL

.I DI -n- AMYL E T H E R

O X Y G E N A T E D

1 8 0 / 6 0 o

I k J I- B U T A N O L

.5 ~ ACETAL

.01 I I I I I I I I I I I I I 0 40 80 120 160 200 240

FIG. 6. RELATIVE RESPONSE RATIOS OF PAIRS OF SPEC- TROMETERS FOR SOME OXYGENATED HYDROCARBONS

be considered as a representative sample of the ions formed. In some way ions of the same formula and mass are treated differently, depending on their origin, upon changing the repeller potential, even though the energy of the ionizing electrons has not been significantly changed.

This might be explained as follows. First, an ionizing electron converts the molecule into an activated complex as theorized by Rosenstock and associates (5). This com- plex dissociates by several mechanisms, each with its own rate constant. These dissociations occur in times of the order of the transit times of ions in the ion source and ion gun. The ~ons dissociating in the ~on gun produce diffuse peaks and are termed metastable ions.

Since a molecule may gain energy m excess of that needed for ~omzatlon, fragment ions of many types result, and some of the fragments may be produced with con-

10

H A L O G E N A T E D

1 8 0 / 9 0 °

• 5 C2H2Br 4

~ ' C 2 H 2 B r 2 ~ - ' , ~ - - CH o, B,

C2CI 4

.05

HA L O G E N A TED

6 0 / 9 0 °

s A~

CH2i

i

~ .5

CHCI 3

C2H2Br 4

H A L O G E N A T E D

1 8 0 / 6 0 °

.5

.I

.01 I I I I I I I I I I I I I 0 40 80 120 160 200 240

FIG. 7. RELATIVE RESPONSE RATIOS OF PAIRS OF SPEC- TROMETERS FOR SOME I-IALOGENATED HYDROCARBONS

VoL. 12, No . 6, 1958

siderable kinetic energy. Fragment ions traveling at an angle to the normal ion path wdl be discriminated against and only partially collected, if at all. This effect depends on the imtial energies and the mass spectrometer condi- t i o n s - s l i t sizes, repeller voltage type of focusing, etc.

In general, changing the mass spectrometer operating conditions will change both the ion transit time and the mtt�l energy discrimination. For example, changing the repeller potential will change both of these factors and such a change would be expected to alter the cracking patterns.

There appears to be a distinct difference in the relative sensitivity of the cracking patterns of the hydrocarbons, oxygenated hydrocarbons, and halogenated hydrocarbons to changes in the repeller potential. Possibly the strongly electronegative nature of the oxygen and halogens is a factor. I t is not obwous why the oxygenated and halo- genated hydrocarbons should show such radical behavior ff one considers only ion transit time and initml energy discrimination as the factors revolved. Some other type of interaction with the activated complex seems to be in- dicated.

Conclusion

The survey of the relative cracking patterns shows that conversion of a complete cracking pat tern from one type of instrument to a pattern stated to another type of instrument cannot be satisfactorily performed. The crack- lng patterns from the different instruments are within an order of magmtude m most cases and are useful primarily

171

for identification purposes. Except in the case of the halogenated compounds, cracking pat tern data may be used, wi th an error usually considerably less than an order of magnitude, for small mass intervals.

The variation in relative cracking pat tern as a function of compound type occurs to a considerable extent, even for one type of mass spectrometer when the instrument parameters are changed. Consequently, care must be used when using patterns from a given spectrometer taken under different conditions, part icularly of magnetic and electric field, in the ion source region.

I t would appear that to have universal cracking pat- terns, one needs to determine the cracking pattern of the ionized molecule (activated complex) at the instant of dissociation as formed m a field-free region. These con- ditions are not used in our mass spectrometers because of sensitivity and resolution reqmrements. I t wdl be diffi- cult to verify a mass spectrum obtained from theoretical calculations until the ideal spectrum is obtained.

Literature Cited

1. Jordan, E.B. and Coggeshall, N. D., ]. Appl. Phys. 13, 539-550 (1942)

2. Hipple, J. A. and Condon, E. U. Phys. Rev. 68, 54-55 (1945)

3. Rosenstock, H. M., Wallenstein, M. B., Wahrhaf t ig , A. L., and Eyring, H. Proc. Nal. Acad. Sci. U.S. 38, 667- 78 (1952)

Submitted June 24, 1958

The Effect of Absorption on the Characteristic Curve of Thick Emulsions

Arno Arrak Belmont Smelting & Refining Works, Inc., Brooklyn, N. Y.

Abstract

A general theorem is presented which makes it poss,ble to express the probability of a certam fraction of grains of an emuls*on of finite thickness d becommg developable as a result of an exposure E, provided that the emulsion absorbs light exponentially accordmg to Beer's law, and provided also that a power series expression ~s avadable for the probability of a single gram becommg developable as a result of the same exposure The theorem allows testing of the relation be- tween exposure and developablhty of grains without the necessity of grain counts on single grain layer emulsions. I t Is apphed to the specific case of the Polsson distribution of exposure to obtain a power series representation for the Polsson distribution to exposure in thack emul- sions. The consequences of th*s formula are to be worked out m a future paper. The usefulness of the theorem ~s not restricted to the absorption of hght quanta only, for ~t apphes to any particles whatso- ever wMch are absorbed according to Beer's law in a medium where photochemical or nuclear reactions are produced by the bombarding radiation

Theorem

If the probability that a silver halide grain will become developable when subjected to an exposure E can be ex- pressed as a power series in kE, where k is a constant,

F (E) = ~=~ [1]

then the probabili ty that a fraction P,i of the grains of an emulsion of thickness d are made developable by the same exposure can be expressed by the series obtained from

equation [1] by mult iplying the terms in the n- th power of kE by the factor

-~N. crd

NoO-8 provided that the emulsion absorbs light in accordance with Beer's law. (No m equation [2] is the number of grains per cubic centimeter of the emulsion and ~ is the combined absorption cross section of a silver halide grain and the surrounding small volume of gelatine).

Proof

Conditions for the validity of Beer's law have been discussed by several authors (3, 4) . When Beer's law holds and the exposure at the surface of the emulsion is E then the exposure at a distance x below the surface is

E= E# I t is assumed here that an average volume element in which the absorption takes place contains one silver hahde grain and its surrounding volume of gelatine and can be described by an absorption cross section or. The distance x must be large compared to the diameter of the volume element in order to make equation [3] valid. To obtain the average probabili ty for the grains of an emulsion as