2542
AMI. Chem. 1993, 65, 2542-2544
TECHNICAL NOTES
Cryogenic Coil for Glow Discharge Sources S. K. Ohorodnik and W.W.Harrison' Department of Chemistry, University of Florida, Gainesuille, Florida 3261 1
INTRODUCTION The glow discharge (GD) has heen developed into an analytical technique for the analysis of metals as well as nonmetals, thin films, semiconductors, insulators, and organic materials. The atomic population in the glow discharge can be measured using atomic absorption, emission, and fluorescence spectroscopic analysis while the ionic population can he sampled by mass spectrometry. The detection limits for these techniques vary from low part-per-million to sub part-per-billion.' A common problem encountered with a glow discharge source is the presence of gaseous impurities, particularly air (Nz, 02.C02) and water vapor commonly present in the plasma even after presputter cleaning of the source. The main source of contamination in the glow discharge is water thatadsorbs onto thecathode anddischargechamher surface when the source is vented for sample exchange. This water contamination can influence the sputtering, atomization, and ionization processes in a glow discharge. Larkins2 observed areductioninatomicabsorptionforvariouselements upon addition of water, with the amount of reduction dependence on the type of sample. Ratliff and Harrison3 have shown that the ion signals of different elements are affected by varying degrees in the presence of water. The reduction in analyte signal due to the presence of water can he attributed to a combination of effects, including inefficient sample sputtering, oxidation of the sample surface, loss of analyte atoms through gas-phase reactions, and quenching of argon metastable atoms that are responsible for ionizati0n.~,3 Therefore, water vapor in the discharge must be eliminated or controlled a t a low level to produce reliable quantitative elemental analyses. Effortstoreduceor eliminate watervapor andother gaseous impurities have included (1)use of ultra-high-purity gases to reduce impurity introduction, (2) application of gettering agents to purify further the discharge gases, (3) frequent baking of the source chamber to eliminate absorbed species, and (4) general maintenance of high-vacuum conditions in the system. Cryogenic cooling of the source chamher is also a method for removing the water from the glow discharge source by physically freezing out the water from the gas onto the source chamber walls. Daughtrey and Harrison' introduced a liquid nitrogen cold finger into a glow discharge source, but cooling was relatively inefficient due to unsatisfactory contact betweentheglasscold fingerandthemetalconductor. Commercial GDMS instruments also have provisions for cryogenic cooling of the source, involving the flow of liquid nitrogen flowing through a cryocell block in contact with the (1) Harrison, W. W.; Barshick, C. M.; Klingler, J. A,; Ratliff, P. H.; Mei, Y. AM^. Chem. 1990,62,943A2. (2) Larkins, P. L. Speetrochim. Acta 1991.468, 291. (3) Ratliff, P. H.; Harrison, W.W.,in progress. (4) Daughtrey, E. H.; Harrison, W . W . Anal. Chem. 1975,47,1024. 00062700/~3/0365-2542)04.00/0
out
Flgure 1. Schematlcdbgram of ihenyogei
discharge Ian source.
coil coupled to the glow
source, resulting in the entire GD source being cooled.5 Cryogenic cooling also allows the glow discharge to be used for analyzing difficult elements that would otherwise melt under normal GD conditions.6 Many different types of glow discharge sources are presently in use without any access to cooling attachments. Given the significant reactions that can result from residual water vapor, accessibilityto asimple, effectivedesign for cryogenic cooling may be of interest to analysts. This paper describes a cryogenic cooling coil that is incorporated with a commercial vacuum flangefor directinsertionintothe GD sourcechamber, resulting in a cooling sink adjacent to the glow discharge plasma. The cryogenic coil can be adapted to many different types of source configurations. In our laboratories, this cryogenic coil has been used primarily in a GD source on a mass spectrometer,but it has also found application on a GD source constructed for atomic absorption and atomic emission measurements.
EXPERIMENTAL SECTION The glow discharge quadrupole mass spectrometer and procedure to acquire the m s spectra have heen previously described.? The GD ion source,constructed from a six-way cross with 2.75-in. flanges,was modified by the insertion of a cryogenic cooling coil as illustrated in Figure 1. The coil is made of '/a-in. stainless steel tubing welded to a double-sided 2.75-in. flange allowing the coil to be on axis with the directinsertion probe (Figure 2). The bellows assembly allows the distance between the coil andthe exit orifice ofthe mass spectrometertobe adjustmi from 0 to 2 em. Liquid nitrogen is introduced into the top ofthe flange from an 80-L self-pressurizing dewar (Cryofab Inc., Kenilworth, NJ) with the flow controlled by a stainless steel bellows regulatingvalve(Nupro,Willoughby, OH). To minimize losses in cooling capacity, the liquid Nt transfer lines, cryogenic coil flange, and surrounding flanges are insulated with CMHC No. 10892 pipe insulation (Industrial Thermo Polymers Ltd., Mississauga,ON, Canada). The flow of gaseousNz is monitored downstream from the cryogenic coil using a rotameter (Dwyer (5) US. Patent No. 4,843,539, 1989. (6) Vieth, W.;Huneke, J. C. Awl. Chem. 1992,634,2968. (7) Bruhn, C. G.; Bentz, B. C.; Harrison, W. W. Awl. Chem. 1918,50, 353.
B 1993 American Chsrnlcal Soobty
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18,SEPTEMBER 15, 1993 2543 Threaded Hole for Connector
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Schematic diagram of the coil flange.
11
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.-0 In
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Flgure 3. Glow discharge mass spectrum of NIST SRM 1241b aluminum alloy without cryogenic cooling. Spectrum taken after a presputter time of 30 min at an average discharge current of 3.6 mA.
Instruments,Michigan City, IN). Pressurerelief valves (Whitey, Highland Hts., OH) are also installed between the dewar and regulating vavle and between the coil and flowmeter. For simpler operation, the pressurized dewar can be replaced by an small insulated reservoir mounted on top of the flange, allowing the liquid nitrogen to flow unregulated through the coil by gravity. Ultra-high-puritygrade argon (LiquidAir Corp., San Francisco, CA) was used in all experiments with an operating pressure of approximately 1Torr. Analytical samples were prepared from NIST standard reference materials 1241b aluminum alloy. Samples were milled into 2-mm-diameter pins with 5 mm of the pin exposed in the discharge. The sample pin was approximately 9 mm from the ion exit orifice of the mass spectrometer. The cryogenic coil was positioned 5-7 mm from the ion exit orifice so that the coil surrounded the sample pin without touching it or the probe. The experimentswere performed with an applied dc voltage of 1000 V resulting in discharge currents between 3 and 6 mA.
RESULTS AND DISCUSSION Even under the best experimental precautions, gaseous impurities are commonly still present in the GD chamber. The cryogenic coil was designed to reduce further certain gaseous impurities such as water vapor, carbon dioxide, and hydrocarbons. To demonstrate the effectiveness of the cooling, an aluminum alloy was chosen as the sample cathode, since the greatest interferences from water and other impurities occur in the mlz range between 10 and 40, potentially obscuring analyte ion signals. Figure 3 is the mass spectrum of the aluminum alloy after sputter cleaning the cathode for 30 min at a discharge current of 3.6 mA. The major species in the spectrum are H3+ and ArH+, both a result of residual water vapor. H3+can arise from the dissociation products of water within the plasma8 or adsorbed on the source walls.g ~~
~
~
50 0
1
Ar
100-
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(8) Loving, T. J.; Harrison, W. W. Anal. Chem. 1983, 54, 1526. (9) Stern, E.; Caswell, H. L. J. Vac. Sci. Technol. 1966, 4 , 128.
10
20
30
40
mh
Flgure 4. Glow discharge mass spectrum of NIST SRM 1241b aluminum alloy with cryogenic cooling. Spectrum taken 30 min after turning on liquid nitrogen flow through the coil at an average discharge current of 6.2 mA. ArH+can be produced from the argon ion reacting with water or hydrogen molecules.lOJ1 The peaks at mlz of 14,28, and 29 are a result of air contamination in the GD source, while the peak a t m/z 12 is probably due to contamination from backstreaming of the GD source roughing pump. The oil vapors present in a vacuum vessel can be cracked when bombarded by the electrons, atoms, and ions in the glow discharge, causing carbon to deposit on the cathode and ion source surface.lz This deposition can inhibit the sputtering rate of the cathode since carbon is sputtered at a lower rate.13 Upon turning on the liquid nitrogen flow through the coil, reduction in gaseous impurities begins almost immediately. After approximately 5 min of liquid nitrogen flow, the water species are no longer detected. After 15 min, the H3+and the ArH+ had been essentially removed. Typically it requires about 20 min of cryogenic cooling to "clean up" the glow discharge to a level satisfactory for trace analysis. The glow discharge, operated at constant voltage, showed an increase in discharge current upon elimination of the gaseous impurities, indicating a drop in plasma resistance. This could be explained by removal of quenching agents in the discharge and/or by changes in surface oxidation of the cathode. A mass spectrum of the aluminum sample taken after 30 min of cooling with liquid nitrogen is shown in Figure 4. The cryogenic cooling removes or reduces many of the gaseous impurities. A comparison of the ion signal ratios with and without cooling is listed in Table I. Nitrogen impurities are not removed, of course, but can be reduced by careful attention to the vacuum integrity. H3+,N2H+, and ArH+ signals drop sharply as the source of their protonation reactions (H2O) is removed. Other argon-containing species such as metal-Ar+, -Ar2+, and -Arz+ were not greatly affected by the cryogenic cooling. There is also an increase in the analyte ion signals (Mg, Al) upon cooling. The behavior of the analyte ion signals upon the removal of water vapor depends partly on their reactivity toward atomic oxygen (a byproduct of water dissociation in the glow discharge). Tantalum, which forms a strong metal-oxide bond, has a higher reactivity toward oxygen than does iron and copper, which exhibit lower bond strengths. The magnitude of increase in the analyte ion signal depends upon the reactivity of that element with residual water vapor.14 When the liquid nitrogen flow is turned off, the gaseous impurities desorb quite rapidly (-3 min) from the coil, (10) Rakshit, A. B.; Warneck, P. J. Chem. Phys. 1981, 74,1981. (11) Knewstubb, P. F.; Tickner, A. W. J. Chem. Phys. 1962,36,684. (12) Konig, H.; Helwig, G. 2.Phys. 1951, 129, 491. (13) Laegreid, N.; Wehner, G. K. J. Appl. Phys. 1961,32,365. (14) Ohorodnik, S. K.; DeGendt, S.;Tong, S. L.; Harrison, W. W. J.
Anal. At. Spectrom., in press.
2544
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
Table I. Comparison of Ion Signal Ratios for Aluminum Alloy NIST SRM 1241b with and without Cryogenic Cooling
ion
mlz
3 12 14 18 24 29 40 41 a
Hs+
c+
N+ HzO+
Mg+ COH+,N2H+ Ar+ ArH+
(ion peak area)/PAl+ peak areaP with difference without cooling cooling (%) 4.06 X 3.30 X 2.14 X 2.02 x 7.96 X 4.86 X 1.35 X 1.70 X
loo 10-' 10-' 10-2
le2 10-' 10-f
loo
1.99 x 2.21 x 1.79 X 3.46 X 8.45 x 8.65 X 1.57 X 1.69 X
10-4
103 10-'
103 10-2
1W 10-* 10-2
-99.99 -99.3 -16.4 -82.9 6.2 -99.8 16.3 -99.0
27Al+peak area without cooling 1 180 627 counts, with cooling
cryogeniccooling is in use. Because the direct-insertion probe becomes very cold upon cryocooling, when the probe is removed for sample exchange, water from the atmosphere immediately condenses on the sample and probe assembly. It is convenient to have available a second direct-insertion probe to reduce additional introduction of water and to allow quick sample turnover. This alternate clean, dry probe is inserted into the GD source with the new sample, while the first probe is dried and prepared for the next analysis. The cryogenic coil has been so effective in our laboratory in controlling plasma conditions that it has become a standard part of all our present sources, whether for mass spectrometry or optical measurements. We would be pleased to provide more detailed plans to anyone wishing to construct their own units.
1365 212 counts.
producing once again a mass spectrum resembling Figure 3. Therefore to maintain the suppression of the impurity ion signals, a constant flow of liquid nitrogen is required. By controlling the flow rate of the liquid, an optimum degree of cooling can be reached. In our GD source we have incorporated a ball valve to allow for easy sample exchange when the
ACKNOWLEDGMENT We are grateful for the support of our research by the Department of Energy, Division of Chemical Sciences.
RECEIVED for review April 15, 1993. Accepted June 9, 1993.
