An examination of the spectrum shown in Figure 1B (sediment sample) showed abundant ions at m/e 215,216,217, and 257 and these, together with possible molecular ions at m/e 460 and 488 suggested that saturated sterols were present (8) and the structures cholestanol TMS ether and stigmastanol TMS ether were tentatively assigned. It is also clear from molecular ions at m / e 458, 470, 472, and 484 that the unsaturated counterparts of cholestanol and stigmastanol are also present and from an examination of the fragyent ions (including those below m/e 200) cholesterol, brassicasterol, campesterol, and stigmasterol were tentatively identified. The tentative identifications made here by probe mass spectra have been shown to be very accurate when a complete GLC and GC-MS study was carried out. This method is suggested as a technique for screening a large number of samples to decide which fractions contain
sterols which merit a fuller investigation by the more timeconsuming conventional approach. As illustrated above, it is also possible to arrive at some preliminary identifications especially where some of the compounds being examined yield fairly unique fragments in their mass spectra. It should be emphasized that the method described here could equally well be applied to many other groups of compounds or their derivatives having appropriate volatility. ACKNOWLEDGMENT The authors gratefully acknowledge gifts of sterols from C. J. W. Brooks, D. R. Idler, and Nobuo Ikekawa. RECEIVED for review November 8, 1971. Accepted February 8,1972. The work was supported by Grants from NASA and NSF.
Direct Volatilization of Inorganic Chelates as a Method of Sample Introduction in Atomic Absorption Spectrometry Brian W. Bailey and Fa-chun Lo Division of Laboratories and Research, New York State Department of Health, Albany, N . Y . I2201
IN EAME SPECTROMETRY the conventional method of introducing the analyte into the flame is by aspiration of a solution of the sample by pneumatic nebulizer. This procedure is not very efficient since, although in both premix and total consumption systems a typical aspiration rate is around 4 ml/min, not more than about 5 per cent of this amount is present in the flame as free atoms (1). In the premix system, most of the sample is insufficiently atomized and condenses on the side of the mixing chamber and is lost to analysis. With total consumption systems, the droplet size is so large that only a very small proportion of the sample is desolvated and atomized in the region of the flame in which measurements are made. It can be readily appreciated that increasing the amount of sample that reaches the flame (in an atomizable form) in a given period of time will increase the sensitivity. As a result, the heated chamber technique (2-4) has been introduced in atomic absorption spectrometry, The solvent is vaporized from the aerosol droplets prior to their introduction into the flame. Sensitivities are significantly improved for most elements, as compared with the cold chamber burner technique. A still more efficient method would be to introduce the sample into a flame in the vapor phase so that it would arrive at the flame in a desolvated form, which would increase the efficiency of atomization in the flame. The prerequisite for such a system is a sample that can be readily volatilized. This precludes the majority of inorganic compounds ; however, a number of complexes with organic ligands are quite volatile (1) G. D. Christian and F. J. Feldman, “Atomic Absorption Spectroscopy,” John Wiley-Interscience, New York, N.Y., 1961, p 91. (2) A. Hell, “Advanced Laminar Flow Burner for Atomic Absorption,” 5th Australian Spectroscopy Conferences, Perth, June, 1965. (3) A. Hell, W. F. Ulrich, N. Shifrin, and J. Ramirez-Munoz, Appl. Optics, 7, 1317 (1968). (4) A. A. Venghiattis, ibid., p 1313. 1304
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
SOLVENT
/
BOLING BURNER
Y R R I E R GAS
T
DRAIN
Figure 1. Apparatus for introduction of volatile sample into a flame as a vapor
(5), for example, the complexes of acetyl acetonate and its fluoroderivatives which hsve been used extensively in gas chromatography studies. To date we have confined our investigations to these compounds. EXPERIMENTAL The apparatus is shown schematically in Figure 1. The burner is.the standard Perkin-Elmer three-slot Boling burner. The mixing chamber was constructed out of a tube of steel 3 in. X 5 in. with a blow-out plug at the lower end. The chamber was fitted with three inlets, one each for fuel, air, and sample vapor and an outlet for waste. The whole mixing chamber was heated with nichrome wire. The sample vaporization chamber was constructed from glass tubing of varied lengths and 1-cm i.d. It was fitted with a Y junction at one ( 5 ) R. W. Moshier and R. E. Severs, “Gas Chromatography of
Metal Chelates,” Pergamon Press, New York, N.Y., 1965.
Species c u I+ Cu(acac)e Cu(tfa)z Cu(hfa)% Cu(acac)z Cu(hfa)e Fea+ Fe(aw4a Fe(hfa)s Fe(t fa) Fdacach Fe(tfa)a Fdhfah
Table I. Relative Sensitivities Obtained by Nebulization and Volatilization of Cu, Fe, and Cr Flow rates Sensitivity, Sample Air. Sarnvle. ppm for 1% absorption Fuel introduction Solvent ml/&n' literlkin Water N" 0.17 5.5 24 GHz Benzene 0.06 Benzene 3.3 30 N or or hexane hexane 0.05 5.5 30 Xylol V 0.03 Xylol 4.5 38 0.03 Xylol Xylol V 4.5 38 benzene 0.03 5.5 41 benzene Water N 0.3 5.5 24 GHz Benzene Benzene 3.5 30 N 0.2 or or hexane hexane 5.5 30 Xylol V 0.2 Xylol 4.5 38 V 3.4 30 0.08 Xylol Xylol Hexane
Cr6+ Water Cr(a~ac)~ Benzene Cr(tfa)a or Cr(hfa)a hexane Cr(acac)a Xylol Cr(tfa)a MIBK Cr(hfa)s Hexane a N = nebulization; V = volatilization.
V N
0.05 0.05 0.05 0.5 0.7
GHz Benzene or hexane Xylol MIBK Hexane
N V V V
0.3 0.2 0.16 0.17
end to enable sample and a carrier gas to be introduced simultaneously. The tubing was insulated with asbestos tape and heated with nichrome wire. The burner-mixing chamber assembly was positioned in the burner compartment of a Perkin-Elmer Model 303 and the signal displayed on a Texas Instruments recorder. All atomic absorption measurements were made with the instrument in accordance with the manufacturer's recommended settings. Two methods of introducing the sample into the vaporization chamber were investigated. The first was by using a peristaltic pump, which was very convenient as it enabled rapid changes between samples. However, with the pump used, the flow was not completely surge-free and this resulted in a noisy signal. The second method was by using a variablespeed motor-driven syringe which proved to be satisfactory and was used in all subsequent investigations. Preliminary experiments aimed at optimizing instrumental parameters were carried out with copper acetylacetonate in benzene. The procedure used was as follows. With the apparatus arranged as described, an air acetylene flame was ignited. The flow of the carrier gas (N2)was then increased to about 1 liter/min and the organic solvent that was to be used in the investigation was introduced into the volatilization chamber. The acetylene flow was then reduced until a stoichiometric flame mixture was obtained. Subsequently, it was found that with the relatively high rates of solvent flow employed it was unnecessary to use an auxiliary fuel, and that the air/solvent flame could be ignited satisfactorily without going through a preliminary step of igniting the air/acetylene flame. RESULTS
The preliminary results obtained with copper acetylacetonate in benzene and a I-foot-long vaporization tube were far from satisfactory, the signal being extremely noisy. This was due to two factors; the first was the choice of benzene as solvent. The boiling point of benzene is 80 OC whereas copper
4.5 4.5 5.5 5.5
Hexane
w
g
o,6-
a
3.5
38 27 30 24 30
5.5 4.5 5.5 5.5
30 38 27 30
FLOW R A T E S f u e l (or sample) 3 . 4 m l / m i n air 30l/min
A
a
0
2
4
6 PPm
8
IO
12
14
cu
Figure 2. Calibration curves for copper acetylacetonate in xylene with a 1-foot-longevaporation chamber acetylacetonate volatilizes at about 148 "C. This differential in temperature results in the solvent being vaporized preferentially and the acetylacetonate is left in the tube where it either vaporizes or decomposes. On the changing to a solvent with a higher boiling point, namely, xylene, a more reproducible signal was obtained. However, the initial signal obtained just after introduction of the sample quickly fell to a lower level. This decline in signal was due to an insufficient heat capacity of the vaporization tube. The initial high rate of volatilization falls off as the tube cools to the equilibrium temperature. However, the results obtained in this manner were quantitative and me3surements could be made at either the initial peak or on the subsequent lower plateau A as can be seen from the calibration curves shown in Figure 2. At this juncture the heat capacity of the vaporization chamber was increased by lengthening the tube to 4 feet. This effectively eliminated the falling off of the initial peak. However, the signal was still too noisy. The more volatile hexafluoroacetyl-acetonate complex was then investigated and gave a far more stable signal. ANALYTICAL CHEMISTRY, VOL. 44, NO. 7 , JUNE 1972
1305
W
v
0.6
/
fuel (or sample) 5.5ml/min
/
E 0.4
. 2
0
4
6
fuel
(or sample) 4.5ml'min z r I/mi,n air . carrier g?s(Npli, i / m i n
8
IO
I2
PPm Fe
Figure 3. Calibration curves for iron acetylacetonate in hexane at different sample introduction rates 0.20
a 16
W V 2
a
0.12
m
a
0
v)
m
0.08
a
0.04 0
k 7 ,
0.4
0.0
1.2
I
O
1.6
)
2.0
'
I
2.4
S
I
2.8
I
,
3.ZL
ppm Fe
Figure 4. Calibration curves for iron trifluoroacetylacetonate in xylene at different sample introduction rates The other complexes investigated were the acetylacetonates (and fluoroderivatives) of iron and chromium. The results confirmed those obtained previously for copper in that sensitivity and precision increased with increasing volatility of the complex. In general, this means that optimum results are obtained with the hexa- or trifluoroacetylacetonate complexes as opposed to the unsubstituted acetylacetonates.
complex being investigated is quite critical. If the boiling point of the solvent is much lower than the temperature of volatilization, decomposition of the complex in the tube occurs which results in a reduction in signal and an increase in the noise level. If the temperature differential was reversed, i.e,, the boiling point of the solvent being much greater than the volatilization/decomposition temperature of the complex, in some cases the signal is again reduced and the noise increased. For optimum results, the solvent should be chosen with a boiling point as close as possible to the volatilization temperature of the particular complex being investigated. Rate of Solvent Flow. All other things being equal, it would be expected that by increasing the rate of solvent flow the signal would be increased accordingly. This was the case only within certain limits. Moreover, the change in signal size was also dependent on the particular solvent used. This phenomenon is probably due in part to insufficient heat capacity of the tube. Thus, when the maximum volatilization rate has been reached for a particular solvent and flow rate, any increase in the flow rate will not cause any change in the magnitude of the signal. An example of this is shown with the behavior of the iron complexes. With Fe(tfa), in xylene, increasing the flow rate from 3.4 to 4.5 ml/min gives a corresponding increase in signal (Figure 3). With Fe(hfa)* in hexane, on the other hand, increasing the flow rate from 4.5 to 5.4 ml/min gives no appreciable increase in signal (Figure 4). The relative sensitivities (ppm for 1 absorption) obtained by introduction of the sample by direct volatilization as compared to aspiration for the various complexes investigated are summarized in Table I. It would be expected that the sensitivities of the signal obtained by volatilization would be considerably greater than those obtained by nebulization for equivalent sample uptake rates. However, with the possible exception of the iron complexes, such increases were not observed. This relative lack of sensitivity can probably be ascribed to the volatilization process used; a volatilization chamber which has a greater heat capacity and a better temperature controller would probably increase the sensitivity considerably.
DISCUSSION
Solvent Effects. The relationship between boiling point of the solvent and the temperature of the volatilization of the
RECEIVED for review July 16,1971. Accepted February 15, 1972.
Argon-Water Mixtures as Reagents for Chemical Ionization Mass Spectrometry Donald F. Hunt* and J. F. Ryan I11 Department of Chemistry, University of Virginia, Charlottesville, Va. 22901
IN CHEMICAL IONIZATION MASS SPECTROMETRY (CIMS) (I, 2), a set of reagent ions is generated by bombarding a suitable gas with high energy electrons (70-500 eV) at a pressure of ca. 1 Torr. Sample molecules are introduced into the mass spectrometer by the usual methods and are ionized by ion-molecule 1
Author to whom correspondence should be addressed.
(1) F. H. Field, Accounts Chem. Res., 1,42 (1968). (2) M. S. B. Munson, ANAL.C H E M .(13), , ~ ~28A (1971). 1306
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
reactions with the reagent ions. To date most CI spectra have been obtained using methane as the reagent gas. When subjected to electron bombardment at 1 Torr, methane affords CH5+and C2H5+in high abundance and these ions function either as proton donors or hydride abstractors toward sample molecules ( I , 2). In general CI(CH,) spectra are characterized by the presence of abundant ions in the molecular weight region (M 1 or M - 1) and relatively few fragment ions at lower mass values. As a consequence, the CI method is ideally suited
+
