ination of Trace I Claude Cherrier and Mithat Nalbantoglu Ceiztre de Reclzerches de la Croix de Berny, Socie‘te‘ de Produits Chimiques, Pechiney-Saint-Gobain, 92. Antony, France
THEUSE of spark source mass spectrometry for trace analysis in solids and powders has become an established technique in the last few years (1-3). Recently, analysis of low-meltingpoint metals and liquids has been described (4, 5). In as much as this technique presents the advantages of being very rapid and sensitive, it is useful to apply to the trace analysis of high purity liquids. With the development of new sample-cooling systems this can now be performed easily.
p L q u i d Nitwen
EXPERIMENTAL
Equipment. The instrument used for this work was an A.E.I. MS 7 double focusing mass spectrometer of MattauchHerzog geometry. The upper part of the source housing was modified to incorporate a sample cooling system which is shown in Figure 1. This assembly consists essentially of a Dewar vessel to the bottom of which is attached a specimen holder made of silver. The specimen, which is contained in a high purity graphite crucible, is placed on the lower part of this holder which is in itself detachable to facilitate sample loading. When liquid Nz is used as the coolant, temperatures of the order of -100” C can be attained within 10 minutes after pumping out, and equilibrium temperature of -130” C can be reached in less than half an hour (Figure 2). Temperature measurements were made in vacuum using a chromelalumel thermocouple. The materials are sparked against a high purity graphite counter-electrode. Manipulators allow the sample and counter-electrode to be positioned in front of the exit slit to facilitate good transmission and minimum sample consumption during the longer exposures. Procedure. MERCURY.The graphite crucible is filled with the test sample and attached to the detachable portion of the silver arm. The whole assembly is cooled to liquid Nztemperature and then attached to the bottom of the Dewar. The source region is evacuated and the sample is usually still frozen by the time a rough pressure of about torr has been reached. The Dewar is now filled with liquid NP and the cooling continued. It is necessary to allow the cooling to continue until rough vacuum conditions because mercury has a vapor pressure of about torr at room temperature. At this point a pre-spark is carried out to clean the sample surface. Then, the source space is connected to a diffusion pump and within 15 minutes a vacuum of about lo-’ torr can be achieved and the analysis performed. (1) R. Brown and W. A. Wolstensholme, Gams Publication (Paris), July-Sept., 1964, p. 231. (2) R. D. Craig, G. A. Errock, and J. D. Waldron, “Advances in Mass Spectrometry,” Vol. 1, Pergamon, London, 1959, p. 136. (3) M. Nalbantoglu. Bull. Sac. Chirn. France, 1965, 539. (4) M. Nalbantoglu, “Advances in Mass Spectrometry,” Vol. 3, Pergamon, London. 1966, p. 183. ( 5 ) M. Nalbantoglu, Chirn. Anal., 48, No. 2, 85 (1966). e
ANALYTICAL CHEMISTRY
‘Tantalum Support
Vacuu-m Nuclear Gmphtte Gucible
Figure 1. Schematic of new sample cooling system
ACIDS. To obtain a continuous and homogeneous spark in the case of other liquids, such as acids, it is necessary to mix them with high grade graphite powder to form a paste before depositing them in the crucible. Apart from this, the procedure is the same as for mercury with the exception that a pre-spark is not necessary. Twelve to fifteen graduated exposures are recorded on one plate. Interpretation of the plates is carried out either by visual method or by densitometry. Densitometry allows a reproducibility of about * 2 0 z to be achieved. The spark conditions usually observed are the following: R F spark voltage, 20 kV; maximum pulse repetition rate, 100 c/sec; pulse length, 100 psec. RESULTS
Analysis of Mercury. We made two plates corresponding to the analysis of two samples of mercury. On the plate representing the analysis of an industrial sample, impurities such as bismuth, copper, iron, zinc, and manganese can easily be seen. These elemental lines are very weak on the plate which represents the mass spectrum of a sample of high purity mercury. The identification of impurities is relatively easy, even where hydrocarbon and isotope lines occur at the same nominal mass, since the instrument has a sufficiently high resolving power. For example, the doublet which can be seen at mass 27 is due to Al+ being resolved from C2H8+.
A
Temperature
30 0
Table I. Analysis of Two Mercury Samples Samples, ppm weight Go 1 Industrial No 2 High purity Impurities 30 Bi 0.05 1 30 Cd Ga 0.1 1.5 Zn
cu Fe Mn Ca K c1 A1
Na
9 2.5 5-6 0.08 0.4 0.4 0.25 0.4 0.8
0.2 0.15 0.6 0.01 0.2 0.1 0.08 0.1 0.1
Table 11. Analysis of Two Phosphoric Acids Phosphoric _ _acids, _ _ _ppm _ ~ Sample No 1 Impurities Sample No 2 170 Bi ... 1500 Pt 2 145 co 5 165 Ni 0.2 10 Fe 6 160 0 Mn 3 Cr 2 13 Ca 8 K
c1
Figure 2. Results using sample cooling system
In many cases we separate also oxygenated compounds from the impurities, for instance, with a source slit of 5.10-2 mm we obtain a resolving power of about 2000 in the low mass region which enables us to distinguish easily between COf and 28Si- and between 02+and 32S-. Table I represents the values of the main impurities in the two mercury samples. The results are expressed in ppm weight. Analysis of Phosphoric Acids. Figure 3 shows the spectra of two samples of phosphoric acid. A line at almost every mass can be seen in these spectra. The most significant lines (PO4H4)+are marked due to ions P+, PO+, PO2+,(PO~HZ)+, on the plate. (POeH4)+and (P20iHb)+ represent the molecular ions 1 H of orthophosphoric and pyrophosphoric acids. These ions will not be discussed here because fragmentation and recombination phenomena are quite complicated in the spark source. The main interest of this study is the detection of elemental impurities. Again, the impurities can be identified because of the resolving power of the mass spectrometer which gives, on the plate, doublets and even triplets at a nominal mass. Rough mass determinations by measurement of the doublet separations and checks on the isotopic abundances give further evidence about the presence of impurities. Quantitative analyses have been made by using, as internal standard, the 31 line due to P+. The concentrations in ppm atomic having thus been obtained, the conversion to ppm weight, with relation LO HnP04of molecular mass 98, was carried out. Table I1 shows the results for 13 impurities. Sample N o 2 is a pure commercial orthophosphoric acid. The specifications for this acid, given by the supplier, for three elements are the following: F e < 10 ppm; Mn < 5 ppm; C1 < 5 ppm. Our results in Table I1 are in agreement with these figures.
+
A1 Na
B
8 4600 4 90 90
4 1.2 2.5
-80 0.01
Table 111. Corrected Results Using Known Internal Standard for Sample No 1 Weight Weight real Impurity measured, ppm value, pprn Bi 212 200 +6 co 182 200 -9 Ni 206 200 +3 Mn 200 (standard) 102 100 $2 B
iz
Sample No 1 was obtained by artificially doping sample No 2 with 100 ppm weight of boron and 200 ppm weight of bismuth, cobalt, nickel, and manganese. The chlorides of these elements were used; this explains the large amount of chlorine in sample No 1. The excessive value of platinum in this sample is due to the contamination from platinum crucibles used for doping operations. Na and K concentrations were reduced by a factor of 5 , to take into account thermal ion production of Na+ and Kf. This factor was determined experimentally in this laboratory (6). The concentrations of bismuth and platinum have been increased by a corrective factor of two. This is roughly the factor determined in our laboratory for the loss of sensitivity for heavy ions with our instrument and the Ilford QII plates we use (6). In doing this we are also in agreement with Bourguillot et al. (7). (6) M. Nalbantoglu, Unpublished data, Centre de Recherches de la Croix de Berny, Pechinep Saint Gobain, 92. Antony, France. (7) R. Bourguillot. A. Cavard, and R. Stefani, C. R. Acad. Sc., Paris, 263, 928 (1966). VOL. 39, NO. 13, NOVEMBER 1967
1641
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w
0
E
Fa
La
1642
ANALYTICAL CHfMlSTRY
~~~
We found lower concentrations for all doped elements when we used the 31 phosphorus line. But by taking as an internal standard one of the doped elements, our results became more accurate. Table I11 gives the corrected values for bismuth, cobalt, nickel, and boron where manganese was used as internal standard. Mass corrections for plate sensitivity were made for bismuth and boron. A further improvement in accuracy might have been obtained by correction for relative ionization efficiencies. The relative sensitivity coefficients for different elements in a spark source are not well known. We believe that they vary not only with electrical sparking conditions, which could easily be reproduced, but that they depend also on other parameters, such as the distance between electrodes, the temperature of the sample, etc. , , ,, and these parameters are different from one instrument to the other. This is the reason why we have not tried to use correction factors for ionization efficiencies proposed by other authors (8). Further work needs to be done in this field. However, Table 111 shows that quite good results can be obtained by this method even without relative ionization sensitivity correction. Analysis of Nitric Acid. The analysis of this acid was carried out in the same way as phosphoric acid. The quantitative analysis was made using 69Co as an internal standard. As we have already seen in the case of phosphoric acid, more accurate results are obtained with a known internal standard. Table IV shows our results compared with the results of spectrochemical analysis of a transistor grade nitric acid (J. T. Baker Chemical Co., Phillipsburg, N. J.), which has been doped with 50 ppm weight of cobalt. (8) B. Chakravarty, H. E. Duckworth. and V. S. Venkatasubramanian. “Advances in Mass Spectrometry,” Vol. 2, Pergamon, London, 1963, p. 128.
~
Table IV. Analysis of Transistor Grade Nitric Acid (The impurities are expressed in ppm weight in relation with HNOJ SpectroBaker Impurity Spark source chemical“ specifications CO (standard) 50 ... ... cu 0.02 (0.01
