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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
Mass Discrimination Effects in a Quadrupole Mass Spectrometer Karl V. Wood, Andrew H. Grange, and James W. Taylor" Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
A method for studying the effects of mass discrimination in a quadrupole mass spectrometer system is described. Results were obtained at various photon wavelengths using photoionization mass spectrometry. Additional results were obtained using electron impact, in order to extend this approach to other instruments and Iaboratorles.
Mass discrimination in ion sampling and variations in ionization cross sections make absolute calibration of a mass spectrometer difficult. Separating these two effects requires knowledge of the ionization cross sections and a qualitative description of the instrumental and chemical effects which cause mass discrimination. These include contributions from the amplifier/discriminator electronics, variations in ion intensity caused by the electron multiplier (detector), the influence of the electric field in the ionizing region, and losses arising from the mass filter itself. In most experiments where ions are mass analyzed to determine the structure of the species present, knowledge of the contributions from mass discrimination is not necessary. However, in determining reaction rates by measurement of the relative abundance of parent and product ion formed ( I , 2 ) this information can be important. Also when one is attempting to determine the true relative concentration of two species in the ionizing region ( 3 ) or the concentrations of species sampled with a supersonic molecular beam ( 4 ) , the effect of mass discrimination is important. For these last two reasons, we have studied mass discrimination effects in our photoionization mass spectrometric system. In the past, other workers have studied mass discrimination associated with specific mass spectrometer components such as the magnetic electron multiplier ( 5 ) , or particular experimental parameters like the magnetic sector (6) or fringing fields in a quadrupole mass spectrometer ( 7 ) . These studies, however, generally have not provided quantitative determination of mass discrimination effects which could be easily related to other laboratories. In this study we attempt to do this by using photoionization to measure the ionization cross sections exactly and thereby evaluate mass discrimination effects in our mass spectrometer system. Then, comparing ion intensities (under electron impact conditions) measured using ion counting and direct current measurement, we attempt t o separate the effect of the counting system on the observed mass discrimination. Besides enabling determination of the effect of the counting system on mass discrimination, the comparison of the photoionization results with those from electron impact was made t o make the results of this study of more general use.
EXPERIMENTAL Our experimental setup has been described previously (8). Very briefly, our apparatus consists of a 0.33-mm nozzle, a 0.62-mm skimmer, and a 1.70-mm collimator axially arranged to form a supersonic molecular beam sampling system. This feeds the ionizing region of a Finnigan (model 1015) quadrupole mass spectrometer utilizing a Johnston Laboratories (model MM-1) 20-stage Cu-Be electron multiplier powered by a Fluke (model 0003-2700/78/0350-1652$01.00/0
408B) high voltage power supply. The counting system consists of a Tennelec (model TC 145) scintillation preamplifier, a Tennelec (model TC 202BLR) linear amplifier, a Mechtronics Nuclear (model 604) discriminator, and a Tennelec (model TC 592P) digital ratemeter. The vacuum pumping in the mass spectrometer chamber has been altered from reference 8 to include an Air Products (model 102) closed cycle helium cryopump. The Sargent-Welch (model 3102C) turbomolecular pump is now used to pump the chamber between the nozzle and skimmer orifices. This pumping supplements the liquid nitrogen pumping coil previously described. The chemicals used in this study include ammonia, 99.9970 purity, Midwest Ammonia Corporation; ethane, 99.99% purity, Matheson Gas Company; 1,l-difluoroethylene (CzH2Fz),99.070 purity, Matheson Gas Company; carbon dioxide, 99.99% purity, Matheson Gas Company; and chlorotrifluoroethylene, (C2F3C1), 99.0'70 purity, Matheson Gas Company. Ternary mixtures of these gases were made in previously evacuated cylinders (carbon dioxide and ammonia were never present in the same mixture). Each species was added separately using a Pennwalt (model FA1601 Wallace and Tiernan gauge to record the pressure (estimated accuracy h0.5 Torr), to a total gas pressure in the cylinder of approximately 760 Torr. Using a needle valve, the mixture was admitted directly into the mass spectrometer chamber of the apparatus. The relative intensities of the molecular ions of the various chemical species were measured at various spectrometer chamber pressures using the ion counting system described previously. The ratio of one molecular ion to that of another was found to be constant for any given pressure. Counting times in the photoionization study ranged from 20 to 100 s, depending on the gas pressure and the wavelength of light. Typically the electron multiplier voltage was set a t 3200 V and the discriminator was set to 0.3 V. In the electron impact study, the molecules were ionized using an electron energy of 70 eV with an ionizing current of 4 FA. Two methods of detection were used, ion counting and direct measurement of ion current from the electron multiplier using a Keithley 417 high speed picoammeter. The photoionization experiments were carried out at the Synchrotron Radiation Center of the University of Wisconsin Physical Sciences Laboratory. A Seya-Namioka type monochromator was used with 300-pm slits and a MgF, overcoated A1 1200 l/mm grating. The FWHM resolution of the monochromator was, therefore, approximately 3 A. The experiments were performed a t three wavelengths: 861 A (14.4 eV), 886 A (14.0 eV), and 1060 A (11.7 eV). These wavelengths were chosen to minimize sample fragmentation. Furthermore, the primary argon resonance, 1066 A (11.63 eV), of a CW microwave discharge lamp with a LiF filter was used to provide a bridge between the synchrotron radiation results and the laboratory, where this line source can routinely be employed. Photoionization cross sections were determined using a triple cathode plate photoionization chamber utilizing guard plates to minimize end effects, which has been described previously (9). Briefly, these measurements were made by taking the ratio of adjacent plate currents il:iz or i2:i3. From a knowledge of the ion collection plate distance, D , the absorption coefficient, p , could be found using Equation I.
From a number density of the gas, n, the absorption cross section, u,, could be found using Equation 2. ug = p / n
1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
Table I. Absorption Cross Sectionsa and Photoionization Yieldsb at Various Wavelengths 861 A
886 A
2.65 x l o - ’ ’ 0.382 C,H, o a 7.96 x 1 0 - 1 7 17 0.687 co, o a 2.08 x 10-17 R 0.208 C,H,F, u, 5.87 x 10.’’ 0.324 77 C,FjC1 u, 8.36 x lo-’7) 0.573 a The standard deviation in ard deviation i n q is + 10%. NH,
u, n
Table 11. Electron Impact Ion Intensity, Cross Section, and Percent Molecular Ion Intensity Ratios
1060 A
2.39 x l o - ’ ? 2.36 x l o - ’ ’ 0.361 0.222 8.54 x 10-17 0.660 5.58 x 10-17 0.504 6.16 x 10.” 2.3 x lo-’’ 0.303 6.48 x lo-’’ 4.24 x lo-’? 0.506 0.298 o, is -r 2.5%. The stand-
= 70,
(3)
For these photoionization yield measurements, the photoionization chamber was used in a single ion collection plate mode by interconnection of all five plates, and a total current was measured. The chamber ends were at the anodic potential t o ensure collection of all ions formed within the chamber.
RESULTS A N D D I S C U S S I O N Two types of experiments were necessary to study mass discrimination effects in the mass spectrometer system. First, it was necessary to measure the photoionization cross sections of the given chemical species a t the chosen wavelengths. Second, it was necessary to record the mass spectrum of a known mixture of these species a t these wavelengths. If there were no mass discrimination, the expected ion ratio of two species in the prepared mixtures could be calculated. T h e necessary information would be the partial pressure of each species in a particular mixture, the photoionization cross sections, and the measured relative amount of molecular ion compared to the total fragment plus molecular ion. The photoionization cross sections for the species under study a t the specified wavelengths can be calculated from the data in Table I. The predicted molecular ion intensity ratio of two species can be calculated a t each wavelength as follows: I1
R a t i o predicted 7 = 12
P, x P, x
LT1
(T,
x x
ion ratio
ratio of the molecular ion intensities
ratio of electron impact cross sections and percent molecular ion to total ion intensity
30144 64144 116144
0.354 0.533 0.112
0.290 0.532 0.121
~~~
The photoionization yield for a sample gas could be determined from the ratio of ions formed:photons absorbed for that gas. For the noble gases, 7 = 1. To determine 7,the pressure of the gas within the chamber was increased until the measured ion current reached a maximum. At this point a ratio of ion current:light absorbed was calculated. The ion current:light absorbed ratio for each sample gas was ratioed to the ion current:light absorbed ratio for Ar at 720 8, to obtain p for the sample gases at each wavelength studied. From the photoionization yield, 7, the photoionization cross section, uI could be found using Equation 3. ‘JL
m / z intensity of molecular ion, total ion intensity component,
m / z intensity of molecular ionz
(4)
total ion intensity component,
where PI = partial pressure of component 1 and P2 = partial pressure of component 2. In reality, the observed ion intensities in a mass spectrometric system include the contribution of all mass discrimination effects. Thus, the measured ratio of two molecular ions does not agree with the ratio predicted from Equation 4. A pairwise comparison was chosen to enable calculation of mass discrimination effects relative to a standard. Carbon dioxide was chosen as the standard reference. T h a t is, the amount of carbon dioxide measured was set equal to the
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--s n
Flgure 1. Mass discrimination as a function of m l z , in photoionization study. Curve A , quadrupole resolution of 75 at m l z 44 and 400 at m l z 116; curve B, quadrupole resolution of 70 at m l z 4 4 and 350 at 116
amount predicted. Therefore, comparisons of predicted and measured ion intensities relative to carbon dioxide ion intensities were used to assess the mass discrimination of each ion relative to carbon dioxide, m / z 44. T h e mass discrimination can then be calculated using Equation 5: Mass discrimination = (11/12measured)/(Il/12 predicted) ( 5 ) This is illustrated in the following example. Consider a mixture consisting of equal partial pressures of C 0 2 and C2H2F2 From the ionization cross sections @ 886 8, calculated from Table 11, a predicted ratio of ion intensities can be calculated, using Equation 4: Ratio predicted I C 2 H 2 F 2 / I C O 2 = 0.62 In the absence of mass discrimination, the predicted ratio should agree with the experimentally measured ratio. When the experiment was performed, however the following value was obtained: Ratio measured I C 2 H 2 F 2 / I C 0 2 = 0.41 Therefore, the measured intensity of C2H2F2relative to COz, was 0.4110.62 or 66% of the expected value. Thus, C2HzF2, m / z 64, is attenuated by 34% relative to CO,. Results of a photoionization study, for a particular resolution, R , ( R = M / m , where M is the mass of the peak of interest and Ah4 is the FWHM divided by the distance separating M from M 1) as seen in curve A of Figure 1, shows the effect of mass discrimination as a function of m / z . In this study, the quadrupole resolution was 7 5 at m / z 44 and 400 a t m / t 116. From curve A of Figure 1 it can be seen that the intensity of the heaviest ion studied, C2F3Cl,m / z 116, is attenuated by about 66% relative to the carbon dioxide ion. This large mass discrimination is the result of several factors. T o assess the contribution of mass filter resolution, the high mass sensitivity of the quadrupole was increased. The resulting resolution was 70 a t m / z 44 and 350 a t m / z 116. The mass discrimination results of this study are summarized in curve
+
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
B of Figure 1. From these two photoionization studies, it can be seen that adjustment of quadrupole resolution has caused a large increase in the C2F,Cl ion intensity relative to the Cog ion intensity. Since all other conditions were unchanged, the large difference between curve A and curve B indicates qualitatively the effect of quadrupole resolution on the relative mass discrimination of different molecular weight ions. Next, an electron impact study was made for comparison with the photoionization results. This was done for two reasons: (1)to separate the mass discrimination effects of the quadrupole and detector from those of the counting system, and (2) to make the results of this study of more general use. In the electron impact study, the same experimental conditions (except for the ionizing source) were used as in obtaining the photoionization results summarized in curve B of Figure 1. Therefore, the mass discrimination of a particular ion relative to carbon dioxide should remain the same. The ratio Z1/Zz predicted for this electron impact study can be obtained from Equation 4 by substituting electron impact cross sections (EICS) for photoionization cross sections. Then combining Equations 4 and 5, the following relationship can be obtained: molecular ion EICS, EICS2
X
I1
total ion I , molecular ion Z2 total ion
Z2
1
-
ratio I , /Z2measured
P,/P2 x mass discrimination (6) where P is the partial pressure of a particular component. Even though the components making up the left-hand side of Equation 6 were not measured, it is possible to solve for the left-hand side since the ratio Z1/Z2 measured, P 1 / P 2 and , mass discrimination are known. The results for ethane, C2H2F2,and C2F3Cl relative to COz are shown in the last column in Table 11. Using the same experimental conditions as for Table 11, but measuring the electron multiplier output current directly (instead of counting ions), the mass discrimination due only to the quadrupole and electron multiplier can be determined using Equation 5 . Mass discrimination can be determined because the ratio of electron impact cross sections and percent molecular ion to total ion intensity tabulated in Table I1 can be used to calculate the ratio I1/Zz predicted. T h e effect of mass discrimination as a function of m / z found in this study is shown in Figure 2 . These results are nearly the same as those found in curve B of Figure 1 under the same experimental conditions and resolution using photoionization and the counting system. This implies that under these given conditions the major contribution to mass discrimination is the combination of the electron multiplier and quadrupole mass spectrometer. The effect of mass discrimination in electron multipliers has been found t o be relatively constant a t the low accelerating voltages (electron multiplier voltage, 3200 V) used in this study ( I O , I I ) . Therefore, this means
Figure 2. Mass discrimination as a function of m l z , in electron impact direct current measurement study, with quadrupole resolution of 70 at m l z 44 and 350 at 116
much of the effect of mass discrimination can be minimized by the appropriate resolution adjustment on the quadrupole mass spectrometer as seen by the differences between curves A and B of Figure 1. In order to utilize the results presented here, it is necessary to make a mixture of a t least two of these components. The predicted molecular ion ratio can be calculated using Equation 4, if a photon source a t one of the wavelengths used in this study is available, since the photoionization cross sections are known accurately and the intensity of the molecular ion:total fragment plus molecular ion can be measured. The actual molecular ion ratio should then be measured. From Equation 5, dividing ratio measured by ratio predicted will give the mass discrimination. If a photon source is not available, the mass discrimination can still be found using the ratio of electron impact cross sections and percent molecular ion to total ion intensity, tabulated in Table 11. With these values the predicted molecular ion ratio can be calculated using Equation 4. After measuring molecular ion ratio, dividing ratio measured by ratio predicted will give the mass discrimination.
LITERATURE CITED H. Milloy and M. Elford, Int. J . Mass Spectrom. Ion Phys., 18, 21 (1975). D. Durden, P. Kebarle, and A. Good, J . Chem. Phys., 5 0 , 805 (1969). K. Cook and J. W. Taylor, submitted to J . Chem. Phys. K. Wood and J. W. Taylor, submitted to Anal. Chem. W. Hunt, Jr., R.McGee, J. Streeter, and S. Manghan, Rev. Sci. Instrum., 39, 1793 (1968). H. Werner, Int. J . Mass Spectrom. Ion Phys., 14, 189 (1974). P. Dawson, Int. J . MassSpectrom. IonPhys., 6 , 33 (1971); P. Dawson, ibid., 1 4 , 317 (1974); P. Dawson, ibid., 17, 423 (1975). G. Parr and J. W. Taylor, Rev. Sci. Instrum., 44, 1578 (1973). A. Grange and J. W. Taylor, submitted to Anal. Chem. M . von Gorkom and R. Glick, Int. J . Mass Spectrom. Ion Phys.. 4, 203 (1970). C. la Lau, in A. L. Burlingame, Ed., "Topics in Organic Mass Spectrometry", Vol. 8 of "Advances in Analytical Chemistry and Instrumentation", Wiley-Interscience, New York, N.Y., 1970, p 93.
RECEIVED for review March 31,1978. Accepted June 12,1978. This work was supported in part by the Office of Naval Research under Contract N00014-75-C-0943. The Synchrotron Radiation Center is operated under Grant No. DMR-74-15089 from the National Science Foundation.
