When EDTA is incorporated into absorbing solutions, the acidity of the solution increases from a pH of 5.4 to a p H of 4.0. Presumably, the EDTA disturbs the equilibrium association with the TCM complex by binding Hg(I1) and liberating additional HC1. To determine the efficiency of collecting SOz at this lower pH value, we investigated the effects of pH. The data, summarized in Table I, indicate that there is no appreciable difference in collection efficiency in the range pH 5 to pH 3 and that pH 4 possibly gives values better than the other two.
Therefore, we can conclude that the use of 0.066 gram of the disodium salts of EDTA in the 0.04M TCM solution is highly desirable, and that values for concentrations of sulfur dioxide can be improved by correcting for the decay in solution.
RECEIVED for review June 15, 1970. Accepted September 1, 1970. Presented at the Forum on Environmental Quality, American Chemical Society, 158th National Meeting, New York, N. Y., September 1969.
Spark Source Mass Spectrometric Survey Analysis of Air Pollution Particulates R. Brown and P. G . T. Vossen Consultant Laboratory, AEZ Scient$c Apparatus Ltd., Manchester, England MOSTOF THE STUDIES carried out in the problem of air pollution have involved the detection and determination of the gaseous oxides of sulfur and nitrogen. Much less information is readily available on the determination of trace elements in particulate matter. In order to acquire the maximum data on particulate analysis, it is necessary to use a method which while having the ability to detect all elements must be rapid in application. Rapidity is essential because of the large sample load expected from an air pollution study network to perform many analyses in a short time. The advent of an electrical detection system fitted to a spark source mass spectrometer ( I ) , allows the analytical chemist to carry out a survey of all elements from atomic number 92 (uranium) to atomic number 3 (lithium) in one scan. The time taken is no longer than 9 minutes, depending upon the concentration range and the accuracy required. The operations involved in changing specimens and in re-establishing analytical conditions in the instrument source take approximately 10 minutes. The preliminary work reported in this note is exclusively on the analysis of air pollution particulates contained in the atmosphere of the City of New York. SAMPLE COLLECTION For the sample, the more usual fibreglass filter pad was discarded in favor of a nitrocellulose filter pad for two reasons: it is easily ashed and has a very low blank, see Table I. The pad was exposed to the atmosphere of New York City for 9% hours and air was drawn through it at a rate of 19 liters per minute by a vacuum operated air sampling device. Thus, the total volume of air drawn through the pad was 10.8 cubic meters. The total weight of sample retained on the pad was measured at 2 milligrams. SAMPLE PREPARATION
The filter pad was placed into a clean silica boat with 0.1 gram of high purity graphite (Graphite type USP supplied (1) R. A. Bingham and P. Powers, 16th ASTM Conference on Mass Spectrometry, Pittsburgh, Pa., May 1968. 1820
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Table I. Data on Elements in Sample and Filter Pad Concentration Ratio in yg per cubic meter sample/blank Elements detected Boron 0.004 NDa ND 0.11 Fluorine >5.5 10 Sodium 11 ND Magnesium 1.1 ND Aluminum ND 63 Silicon 5 1.1 Phosphorus 2.3 ND Sulfur 0.28 ND Chlorine 5 2.8 Calcium 110 40 Potassium 0.23 100 Titanium 0.30 240 Chromium 20,000 1.9 Vanadium 100 0.07 Manganese 22 2.4 Iron ND 0.007 Cobalt 114 0.32 Nickel 250 0.26 Copper 133 1.1 zinc 66 0,005 Arsenic 250 0.12 Bromine 6 0.05 Strontium ND 0.004 Zirconium 25 0.01 Molybdenum ND 0.07 Tin 57 0.02 Barium 5,000 4.3 Lead a ND. Not detected in nitrocellulose blank.
by Ultra Carbon Corporation, Michigan, USA). One milliliter of high purity ethyl alcohol was added as a wetting agent and also 100 pl of an aqueous solution of silver nitrate containing 5 pg of silver as internal standard. The alcohol was ignited and the nitrocellulose slowly burned; the combustion was quite controlled and slow because of the combined effect of the alcohol and water mixture. Finally, to ensure elimination of any hydrocarbon binding compounds, the boat was placed in an oven at 450 "Cfor one hour. The resulting mixture was transferred to a desiccator to cool and then thoroughly ground in a clean agate vial with
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
Figure 1. Part spectra of air pollution sample an agate ball pestle. After grinding, portions of the sample were compressed onto a support substrate of pure carbon. The resulting sample tipped rods ( a/8-inch length, a/az-inch diameter) were free from contamination because they were completely encapsulated in polythene during the pressing operation (2). Contamination with silicon may occur during the grinding process since this is carried out in agate. However, in this case it was not detected in the nitrocellulose blank, and blank values for most elements are relatively small, ratios of samplelblank values are given in Table I. PROCEDURE
The sample electrodes were mounted in the source sample clamps SO that the sample tips formed the analysis gap in front of the first slit. A vibrator takes the place of the micromanipulators used for manual adjustment of the electrode gap. A more stable ion beam is obtained by strict control of the electrode gap; variations of gap are minimized by choice of settings of operating parameters. Emission is also stabilized by the vibration of the device assisting in the removal of particles of sample which may otherwise tend to short out the gap. Use of the vibrator also provides a method for adjusting the electrode gap by an electrical signal rather than mechanical movement. By superimposing a direct current level on the alternating current level, the gap may be widened or narrowed. This principle is used by automatic spark control (3). The right-hand sample clamp was mounted to the first
slit mounting plate, maintaining the distance between the spark discharge and the first slit at a constant value. Five scans were recorded with a multiplier gain of IOs which allowed a limit of detection of 0.004 pg per cubic meter for lead. Choice of multiplier gain allows the analytical chemist to select the dynamic range of concentration to be examined-in this case the limit of detection was in the nanogram range for all elements. The limit of detection could be reduced by taking larger sample weight and increasing the multiplier gain; then a limit of detection of 10 picograms is attainable. A scan t h e of up to 9 minutes allows examination of every element in the periodic table, excluding hydrogen and helium. The spark excitation parameters employed were as follows : rf voltage, 30 kV; spark pulse repetition rate, 1000 pulses per second; spark pulse length, 100 microseconds. The ion extraction voltage used was 20 kV. The use of a cryo-absorption pump, which has been described by Harrington et al. (4), greatly reduced the formation of complex ion species of the type XO* and XOH. APPARATUS
All scans were taken with a spark source mass spectrometer type MS702 (AEI Scientific Apparatus Ltd., Manchester, England) fitted with electrical detection accessories. A vacuum air sampling unit of the Department of Air Resources of the City of New York was utilized to collect the sample. The sample electrode moulding die was supplied by AEI Scientific Apparatus Ltd. RESULTS AND DISCUSSION
(2) R. Brown, W. J. Richardson, and H. W. Sornerford, 15th ASTM Conference on Mass Spectrometry, Denver, Colo., May 1967. (3) R. A. Bingham, P. Powers, and W. A. Wolstenholme, 17th
ASTM Conference on Mass Spectrometry, Dallas, Texas, May 1969.
Part of the spectrum is shown in Figure 1; the concentration range is from 0.004 to 4 pg per cubic meter for the lead (4) W. L. Harrington, R. K. Skogerboe, and G. H. Morrison, ANAL.CHEM., 37,1480 (1965).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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isotopes. Concentrations of the various elements were determined by measuring the height of a peak of an isotope of each element and the peak due to the mje 107 isotope of silver, the internal standard. Measurement was made conveniently by the use of a specially constructed 3-cycle log scale. The relative sensitivities of some elements may differ from that of silver; consequently, the absolute values of concentration of these elements may be in some error. Calculation of element concentration was made using the following expression, where weight of silver internal standard was 5 kg and volume of air samples was 10.8 cubic meters. Ai
I,
5x-x-xAs
10.8
AtWti AtWts
=
Concn in Mg per meter3
(1b
where Peak height impurity isotope Peak height lQ7AgtIsotope abundance of 1QVAg 13 = Isotope abundance of impurity isotope I, At Wti = Atomic weight impurity element At Wt, = Atomic weight of silver. AI A*
= = =
Twenty-eight elements were determined in this analysis; their concentration and the ratio to blank levels in the nitrocellulose are given in Table I, where all concentrations are expressed as micrograms per cubic meter of air. The precision is estimated to be h 3 O x standard deviation, based on previous analysis of elements in a graphite matrix. Higher precision may be obtained on specific elements of interest by using a peak switch and static integration method of analysis (31, but this was not used in this preliminary survey work. The values for ratio of concentration of elements in the sample and the blank provide a guide io the feasibility of the accurate determination of elements on a nitrocellulose filter. It is felt that unless the ratio is equal to or greater than 5 ,
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slight variation in composition in batches of nitrocellulose could be significant. Previous analyses on air pollution carried out in the author's laboratory have shown that local industry has a marked effect on the composition of air pollution particulate samples. The elements lead, bromine, and vanadium are always detected in appreciable amounts in city atmospheres, probably as combustion products of fuels and oils in vehicular traffic. The preliminary work reported here shows the feasibility of analysis of pollution particulates, and the relative ease of sample preparation and production of spectra. No account was taken ofrelative sensitivity of the elements in the matrix or indeed of possible element losses during the combustion. Repeat scans showed no detectable losses of volatile elements during sparking of the sample but there is a considerable risk that this could occur with very volatile elements. Further work will involve the determination of mercury, and selective volativity will be investigated at that time when the authors aim to increase the rate of sample throughput and speed of spectral interpretation by on-line data acquisition and processing. Low resolving power was used so as to provide the greatest number of ions at the collector, this did not impose any serious limitations due to interference although carbon was used as support material. Molecules of carbon do not cause any serious interference in spite of their prolific production, uninterfered isotopes of the elements are usually available. Hydrocarbon interference is virtually eliminated because ofthe use of cryo-absorption pumping in the source region. ~
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The authors are indebted to the Department of Air Resources of the City of New York for providing the sample and authorizing the publication of data.
RECEIVED for review June 12, 1970. Accepted August 28, 1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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