f
a
hr ectro
ica
Donald M. Schoengoldl and Burnaby Munson Department of Chemistry, University of Delaware, Newark, Del.
CHEMICALIONIZATION MASS SPECTROMETRY (or perhaps “ion-molecule reaction mass spectrometry”) is a recently developed technique of analytical interest for the production of mass spectra through gaseous ionic reactions ( I , 2). In this technique, primary ions are generally produced by electron impact in a gaseous mixture at pressures as high as a few Torr. The gaseous mixture consists of the reactant gas and the analytical sample, normally in a reactant gas/sample mole ratio of 100 to 1 to 1000 to 1. Because of the large excess of reactant gas virtually all of the primary ions are produced by direct ionization of the reactant gas, not the analytical sample. These primary ions will undergo reactive collisions with the bulk reactant gas and perhaps produce other ions. To be a suitable reactant gas, a compound must produce a set of ions which does not react with the bulk reactant gas to give further products; i.e., the distribution of ions must achieve a substantially constant value as the pressure or reaction time is increased. To illustrate:
That is, the major (9Ox) primary ions produced by high energy 4 2 3 5 eV) electrons in methane, CH4+ and CH3+, react rapidly with the major component, CH4, to give CH5+ and CzH5+. On the other hand, CHs+ and C2H5+ do not react with methane to produce any other ions; therefore, they may react with the small amount of analytical sample to produce a set of ions which is characteristic of the sample. CHa+ and CzH5+react by proton and hydride transfer to give (MW 1)+ and (MW - 1)” ions which may dissociate further. This distribution of ions is the chemical ionization (CI) mass spectrum of the sample and does depend on the ions of the reactant gas. CI mass spectra are frequently less complex and easier to interpret than electron impact mass spectra ( I , 2). Application of the technique of CI mass spectrometry to gas chromatography-mass spectrometry (GC-MS) can be used to avoid some of the problems inherent in normal GCMS work. The effluent from a gas chromatograph can be used directly in CP mass spectrometry if the G C carrier gas is suitable as a CI reactant gas. This technique enables GC-MS to be accomplished without a molecular separator and eliminates the restriction imposed by the separator that the carrier gas be helium. Recently, two other combinations of chemical ionization mass spectrometry and gas chromatography have been re-
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i
Present address, Sun Oil Co., Marcus Hook, Pa.
(1) M. S. €3. Munson and F. H. Field, J. Amer. Chem. Soc., 88, 2621 (1966). ( 2 ) F. H. Field, Accounts Chem. Res., 1,42 (1968).
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ported (3, 4). Both of these reports concerned quadrupole mass spectrometers and used methane as the carrier gas. EXPERIMENTAL
The mass spectrometer which was used in these experiments was a Bendix Model 12 TOF instrument modified for high pressure work and operated in the pulsed mode with variable time delay (5, 6). An Aerograph dual column, temperature programmed gas chromatograph was connected to the mass spectrometer by a system of copper tubing and Swagelok fittings. A Nupro valve, Model “My’ Cross Pattern Fine Metering Valve, was adjusted to allow sufficient carrier gas and sample into the source of the mass spectrometer to produce the desired pressure, 0.01 to 0.1 Torr. The bulk of the material was exhausted to the atmosphere. Helium and methane were used as carrier gases. The source pressure was varied from 0.01 to 0.1 Torr with ionic residence times as long as 10 psec. A simple circuit was designed to start the scan of a mass spectrum after each GC peak reached a given height. In these experiments, the ratio of sample to carrier gas was about 0.02 with 1 pl of liquid sample injected into the gas chromatograph. Because this concentration of sample is higher than has generally been used in CI studies, direct ionization of the sample and secondary reactions of sample ions with sample might occur. Consequently, the material was diluted by adding an increased flow of carrier gas through the reference column to the gas flow from the sample column and cell. This combined sum was the total effluent which was sampled. The sample to carrier gas ratio was less than 0.005 in the final mixture. RESULTS AND DISCUSSION
The majority of the reported chemical ionization mass spectra have been obtained with methane as the reactant gas. Consequently, methane was used as the carrier-reactant gas. At a source pressure of 0.12 Torr, with ionic residence times as large as 9 psec, CI spectra were obtained for several compounds. Where comparisons are possible, the spectra are in reasonable agreement with the other data ( I , 2, 7). 1)” and Simple aliphatic ketones give primarily (MW acyl ions; esters give (MW I)+, protonated acid ions, and acyl ions; aromatic compounds give primarily (MW l>+ ions; and low molecular weight alcohols give (MW l)+, (MW - l)+,and alkyl ions. Figure l a shows a simple, well-resolved two-peak chromatogram of methanol and ethanol using methane as the
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(3) G . P. Arsenault and J. 3. Dolhun, Paper presented at the 18th
Conference on Mass Spectrometry, San Francisco, Calif., June 1970. (4) Marvin L. Vestal, Paper presented at the 18th Conference on Mass Spectrometry, San Francisco, June 1970. (51 . , C. D. Miller. T. 0. Tiernan. and J, €I. Futrell, Rev. Sci. Ifiszstrum., 40,503 (1969).’ (6) ~, C. W. Hand and H. von Wevssenhoff. Can. J. Chem., 42, 195 (1964). (7) John Michnowicz, unpublished data from these laboratories.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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3
Scan
Scan
mle 32 31
30 29
mle
Scan
60 59 58 57
43 42 41 39 31
Spectra with CHq
CzHaQH Scan 3 Scan 4
CHiOH
mie 29 31 33 41 43 45 47
Scan 1
Scan 2
100 16 25 9 6
100
... ...
100
...
14 21 4
. . a
5 6 6 6
6 ,..
...
Spectra normalized to largest peak
=
100
... ...
7 7 9 7
100.
carrier gas. Table I shows the simple chemical ionization mass spectra (CHd reactant gas, mje 2 29) obtained during two time intervals on each peak. Methanol is easily recogI)* and (MW - 1)+ peaks at mje = nized by the (MW 33 and 31, which are characteristic of low molecular weights 1)" and (MW alcohols. Ethanol is recognized by (MW 1)+ peaks at nile = 47 and 45. The ions at mje = 29, 41, and 43 which are present in CHd alone are useful as mass markers and are disregarded in CI spectra. The ion currents for the samples are reported relative to the ion current for CzH5+simply for convenience in this Table and lo indicate the extent of reaction of and C2H6+ with the alcohols. At present, no quantitative significance can be attached to the absolute values of ion currents or peak heights. The limits of detectability depend upon the nature of the chemical ionization spectrum as well as the operating parameters of a particular system. We have not yet attempted to optimize these parameters; however, even under these conditions about 0.1 p1 of sample could be identified. There is no intermingling of material within the source of the mass spectrometer for these widely separated chroma1)+ to (MW - 1)+ tographic peaks. The ratios of (MW ions for the duplicate scans of a chromatographic peak are sufficiently close (1.56 and 1.50 for methanol and 1.00 and 0.87 for ethanol) to allow identification of the material without undue concern about when the spectra are taken during a chromatographic peak.
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n-CaH?OH Charge exchange0 API No. 284" 10 ... 15 ... 5 ... 3 4
13 10 6 100
... 21 8 9
34
86 a American Petroleum Institute Project 44, ref. IO. b Reference 9. Reference 8. Spectra normalized to largest peak = 100.
Figure 1. Gas chromatographic peaks for mixtures with methane as carrier-reactantgas
Table I.
Spectra with He GHaOM Charge API No. 2 8 2 ~ exchangeb 72 1 100 4 a 3 42 100 Table XI.
This work 38 101) 10
70
This work 12
14 17 10
18 20 23 10 100
Figure l b shows an unresolved peak of acetone and ethanol on a gas chromatograph with methane as the carrier gas. The column conditions and components were chosen for this purpose. The CI mass spectrum of scan 1 was a predominant peak at mje = 59 which is characteristic of acetone. In scan 2, both ethanol and acetone could be identified by (MW 1)+ions at mje = 47 and 59. For this and several other composite peaks of simple compounds, the composite nature of the chromatographic peak and the identity of the compounds could be determined by obtaining the chemical ionization spectra (CH,) twice during a peak. Multicomponent peaks obviously present further problems. The results obtained with helium as a carrier and reactant gas were pleasantly surprising. From previous data on charge exchange reactions of He+ (8, 9), it was expected that the spectra obtained with He as the reactant gas would be analytically useless because of virtually complete dissociation of the molecular ions produced by charge exchange reactions with He". The spectra which were obtained, of which Table I1 is representative, resembled very closely the conventional electron impact mass spectra of the compounds (10). The relative abundance of He+ is unreliable and not given in the table. The observation that spectra were not very sensitive functions of either the total pressure within the source or the residence time of the ions also suggested that the spectra were not caused by ion-molecule reactions. For comparison with our data on methanol and propanol, the charge exchange spectra obtained with #e+ and conventional 70 eV electron impact spectra are shown in Table 11. It is obvious that the spectra cannot be produced by charge exchange reactions of He+. While there may be some contribution of charge exchange under these conditions of relatively low pressure, the differences in spectra between our work and the other electron impact data are probably attributable to different instruments.
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(8) P. Wilmenius and E. Lindholm, Ark. Fys., 21,97 (1962). (9) E. Pettersson,ibid., 25,181 (1963). (10) Catalog of Mass Spectral Data, API Research Project 44,
Carnegie Institute of Technology, Pittsburgh, Pa.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
For a mixture of reactant and sample in the sources of the mass spectrometer, two competitive electron impact ionization processes can occur : Reactant & Ions (He+, or CHI+, etc.)
(2)
Sample $- Ions
(3)
The relative importance of each of the reactions is determined by the electron impact cross section for ionization. The cm2; ionization cross section for He is about 0.4 X cm2(11, 12). CH4, 4.7 X 10-le cm2; and acetone, 10 X Since the ratio of ionization cross sections of sample/He is much greater than the ratio of cross sections of sample/CH*, direct ionization of the samples will be more likely with He as a carrier gas than with CHI. The spectra of the same compounds obtained with the two reactant-carrier gases, He and CHI, are very different from each other as may be seen by comparing the spectra for methanol in Tables I and 11. CHI as a carrier gas gives ions resulting from proton and hydride transfer and subsequent decomposition, (MW l)+, (MW - I>+, and lower frag-
+
(11) F. W. LamDe, J. L. Franklin, and F. H. Field. J. Amer. Chem. soc., 79,612911957). (12) J . A. Beran and L. Kevan, J. Phys. Chem., 73,3866 (1969).
ment ions. He as a carrier gas gives ions resulting from predominantly electron impact ionization or charge transfer, MW+, and fragment ions. The work done with methane on well resolved and composite chromatographic peaks was repeated with helium as the carrier gas and comparable results were obtained. Further work on these systems which is planned includes studying the reproducibility of the system, the effects of operating parameters, and the possibility of quantitation. ACKNOWLEDGMENT
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society and to the University of Delaware Research Foundation for partial support of this research. One of us (DMS) is grateful to the Sun Oil Co. for a leave of absence and support during part of this research. The use of this method is covered by patents pending by Frank Field and M. S. €3. Munson. This patent is owned by Esso Research and Engineering Company with. exclusive manufacturing rights to Scientific Research Instruments Corp,, Baltimore, Md.
RECEIVED for review June 23, 1970. Accepted September 14, 1970.
Determination of lood Ammonia by
pectrometry
Robert N. Hager, Jr.,1 David R. Clarkson,* and John Savorys Department of Aerospace Engineering, Unicersity of Florida, Gainesville, Fla.
A VARIETY OF METHODS have been described for the measurement of ammonia in blood. Most of the procedures have involved a preliminary separation of ammonia from blood either by diffusion (1-8) or by ion exchange chromatography (9,IO). Following this separation the ammonia has been determined by volumetric titration ( I , 6, 8)) coulometry (5), or colorimetry (3, 4 , 7-10). Okuda et al. (11) measured ammonia directly on a protein free filtration of blood using an indophenol colorimetric method, and some modifications of this procedure have been reported (12, 13). Enzymatic 1 Present address, A.B.A. Industries, Inc., P.O. Box 517, Pinellas Park, St. Petersburg, Fla 33565 a Present address, University of Pittsburgh Medical Center, Pittsburgh, Pa. a Present address, Pathology Department, University of Florida, College of Medicine, Gainesville, Fla.
(1) E. J. Conway and R. Cooke, Biockem. J . , 33, 457 (1939). (2) E. J. Conway, “IMicrodiffusion Analysis and Volumetric Error,”
5th ed., Crosby Lockwood, London. (3) D. Seligson and K. Hirahara, J. Lab. Clin. Med., 49, 952 (1957). (4) P. V. D. Burg and H. W. Mook, Clin. Chim. Acta, 8,162 (1963). ( 5 ) G. D. Christian and F. J. Feldman, ibid., 17, 87 (1967). (6) B. A. Tobe, Can. Med. Ass. J., 84, 767 (1961). (7) A. F. K. Buys Ballot and C. Steendijk, Clin. Chim. Acta, 12, 55 (1965). (8) S. Gangolli and T. F. Nicholson, ibid., 14, 585 (1966). (9) D. J. Foreman, Clin. Chem., 10, 497 (1964). (10) S. G. Dienst and B. Morris, J. Lab. Clbt. Med., 64, 495 (1964). (11) H. Okuda and S. Fujii, Saishin Igaku, 21, 622 (1967). (12) H. McCullough, Clin. Chirn. Acta, 17,297 (1967). (13) H. Leffler, Amer. J. Clin. Parhol., 48, 233 (1967).
procedures applied also to protein free filtrates have been described (14,15). The method described here involves the technique of derivative spectrometry and differs from all previous methods in that a measurement is made directly on molecular ammonia. Ammonium ion is converted to ammonia by adjusting the sample to pH 11. At 25 “C,total ammonia nitrogen then consists of 98% ammonia and 2 % ammonium ion. The ammonia dissolved in the sample contained in a closed vessel establishes an equilibrium with ammonia in the vapor state. At low concentrations, the equilibrium vapor pressure is proportional to the concentration of dissolved gas in the fluid in accord with Henry’s law. The ammonia vapor is measured using ultraviolet derivative spectrometry. The theory and operation of derivative spectrometers have been described previously (16-18). The second derivative spectrometer used in the present study has a single light beam which passes through the analyzing cell and is directed upon a radiation detector. The wavelength of this beam varies sinusoidally in time about a center value, the amplitude of wavelength modulation being approximately equal to the width of an ammonia absorption band (typically 20
A).
(14) K. L. Reichelt, E. Kvamme and B. Tveit, Scand. J. Clin. Lab. Inaest., 16, 433 (1964). (15) M. Rubin and L. Knott, Clin. Chim. Acta, 18,409 (1967). (16) E. C. Olson and C. D. Alway, ANAL,CHEM.,32, 370 (1960). (17) G. Bonfigliolig and P. Broveto, Appl. Opt., 3, 1417 (1968). (18) D. T. Williams and R. N. Hager, Jr., Appl. Opt., 9, 1597 (1970).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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