Continuous Mass Spectrometric Determination of Concentration of Particulate Impurities in Air by Use of Surface Ionization William D. Davis General Electric Co., Research and Development Center, P.O. Box 8, Schenectady, N.Y. 12301
The average concentration of particulate impurities in air is determined by impinging the particles on a heated Re ribbon and analyzing the resulting ion current with a small magnetic sector mass spectrometer. Oxygen in the air raises the work function of Re to about 7.2 eV a t 1000 K allowing analysis of elements with ionization potentials as high as 8 eV. In favorable cases, concentrations of g/m3 can be detected. By collecting the particles on the ribbon for short periods and then flashing the filament, increased sensitivity can sometimes be achieved. Natural air particles are analyzed for Li, Na, K, Rb, Cs, Sr, U, Pb, Cr, and Cu. Large amounts of organic compounds are detected, but the mixture is too complex to positively identify individual components. An instrument for the analysis of single particles by use of surface ionization was described previously ( 1 ). By directing a high velocity stream of air against an orifice, an aerosol beam is formed which is directed against a heated Re ribbon in a mass spectrometer ion source. As each individual particle hits the ribbon, a burst of ions is produced which is then focused and analyzed. The number of ions in the burst is a measure of the number of atoms of that element in the particle. If analysis of single particles is not necessary, one can lower the temperature of the ribbon until the desired degree of signal integration is obtained and measure the average ion current produced. The concentration in the air can then be estimated from the particle efficiency (fraction of particles sampled that hit the ribbon and are analyzed), the analyzer transmission (fraction of ions produced at the ribbon which arrive a t the detector), and the ionization efficiency for the particular element. An advantage of this technique is that the lower operating temperature produces a higher work function surface and hence increases the ionization efficiency for elements with a high ionization potential. In some cases, there is the additional advantage that the background level is reduced. This article describes the results obtained by use of this technique.
Results An example of the results obtained for Li in ambient laboratory air is shown in Figure 1and for P b in Figure 2. Before the start of each recording, a filter to remove particulates is inserted in the sampling line to show the background signal of the instrument. A summary of some representative results obtained by this method is shown in Table I. The concentrations listed were calculated with a particle efficiency of 0.2% and an analyzer transmission of 4%.The ionization efficiency was estimated from the Saha-Langmuir equation and the measured work function of the Re ( 1 ) . The air flow was 24 cm:'/s. The background levels for high mass elements such as P b and U were caused primarily by organic compounds in the vacuum system and hence could be improved by using more care to obtain a clean system. The backgrounds for Na and K were caused primarily by the elements themselves present in the filament material. At the low temperature used, the alkali metal was not diffusing to the surface of the Re in appreciable quantities, but instead volatile Re oxides were being formed which exposed the alkali metal. For example, the K+ back-
ground under vacuum or with NZ applied to the sample inlet was less than 1ion per second but with air applied to the inlet, the background immediately rose to about 20 ions/s. The Ca, Ba, Sr, and Cr backgrounds appeared to be caused primarily by evaporation of compounds of these elements from nearby surfaces. The surfaces were undoubtedly contaminated by the large amounts of pure compounds of these elements used during the preliminary phases of this study, and the backgrounds are probably not representative of a clean ion source. Similarly, the high Cs background is due to the use of CsNO:j during bakeout to activate the electron multiplier. In addition to the instrumental backgrounds listed, interference can also be produced by the particles themselves. Besides the obvious case of interferences from ions of the same mass (organic ions being the most troublesome here), there exists the possibility of ions of different mass being scattered or reflected into the mass position being measured. For natural particles the most troublesome offenders are Na+ and K+. The degree to which this broad background of scattered ions interferes with the analysis depends on the proximity of the mass desired to the masses of K + and Na+. For the rather modest mass analyzer used in this study, the Na+ and Kf ion current was attenuated by a factor greater than lofioutside the mass range 20-50. The ambient levels in laboratory air shown in the last column were taken at random intervals of time and do not necessarily represent comparative concentrations of elements for any one time. As an example of a comparative analysis, the concentrations of Li, Na, K, and P b were measured over a short interval of time as 6,30,50,and 700 ng/m3, respectively. The condensation nuclei counter indicated 17 000 particles/ cm3 a t this time.
Analysis by Particle Collection Technique For some elements, lower concentrations can be measured by turning off or reducing the current to the ionizing ribbon, collecting particles on the cooler surface for about a minute or longer, and then rapidly reheating the ribbon to obtain a burst of metal ions. This technique is especially applicable to FILTER ON
I
FILTER LITHIUM IN AIR OFF n-SCALE CHANGE i loo0
YASS 7
IO - 8 l d o A c
Y
8-
3
6 lo-' A = _--.004pq LiIm'
c
2
i
6-
3
4-
I
I 0
I 25
50
75
SECONDS
Figure 1. Measurement of
average concentration of Li in laboratory
air Multiplier gain = 6 X lo6
Volume 11, Number 6, June 1977 593
those elements like P b or U where the background is caused primarily by organic compounds. About 1 or 2 orders of magnitude gain in signal-to-noise level can be obtained, probably because interfering organic compounds are too volatile to be absorbed. The results obtained for P b by completely turning off the heating current are shown in Figure 3. A small burst of ions (presumably organic) is obtained even for the filter on, but even so the background burst is a factor of 1200 less than the Pb+ burst, indicating an equivalent instrument background level of about 0.06 ng Pb/m3, about a factor 10 improvement over constant temperature operations. Better results are usually obtained by lowering the temperature of the ribbon to some intermediate temperature which is sufficiently cool to retain the metal atoms but hot enough to oxidize or prevent condensation of the organic compounds. Figure 4 for U shows that by using 600 "C as the lower temperature, the organic interference can be essentially completely eliminated. The organic ions continue to evolve at the lower temperature. From this figure, one can estimate that the instrumental background is roughly equivalent to 0.0003 ng U/m3. The burst of UO2+ is roughly proportional to collection time. This mode of operation should also be helpful with other types of background problems where the interfering element or compound is more volatile than the desired element (for example, if scattered K + ions are interfering with the measurement of Ca+ at mass 40). The increase in signal level alone that can be achieved with collection techniques is potentially useful, but at the present stage of instrument development, detector noise is not a significant problem.
Environmental Measurements Because the equipment was not designed to be portable, measurements of environmental samples were confined to laboratory air. The building is well ventilated, however, and when analysis of the outside air was desired, the windows were opened. The following observations are given to indicate the potential usefulness of this method of analysis. Lead. Except for the alkali metals, lead was the most prominent metal impurity observed in spite of the fact that its ionization efficiency is only about 3% and the laboratory is located in a semirural area. The P b concentration was typically about 0.1 fig/m3, while the average urban concentration in the U.S. has been stated as about 0.8 fig/m" (2). It was interesting to note, however, that the concentration
FILTER ON
12yxlO'A
Table 1. Concentrations of Several Elements in Laboratory Air IP,
Re temp,
Element
eV
OC
Cs
3.9 4.2 4.3
600 600 600 600 840 1500 1070
Rb
K Na
5.1 5.4 5.7 6.1
Li
Sr U
Background Ievel,a ng/m3
% ionized
100 100 100 100 100 100
0.002
50
0.003
2 0.05 0.5-200
0.0008 0.0004 0.0004 0.0001
0.4-40 0.1-6 10
