1545
Anal. Chem. 1981, 53. 1545-1546
Table 11. Detector Response at the Minimum Background Currenta response for ultraresponse for highhigh-purity helium purity-helium ___ tested 1 2 3 4 5 6 7 8 9 10 11 1 2 Ar -i- 0 , 21 19 20 I 9 18 16 18 1 2 17 21 20 18 Na CH,
co a
18 1 7 14 1 6 14 12 12 1.3 13 19 16 14 32 31 33 31 31 28 29 26 31 34 31 30 53 50 53 !jO 50 46 47 47 52 56 52 49
Peak area in counts x 1 0 - ~ . --
to the ultra-high-purity helium. However, the high-purity helium grade may conkhi a concentration of impurities greater than that at minimum background current. A typical analysis for impurities in high-purity grade helium as provided by the producers is 1 ppm of each of CHI, Oz, Ar, and GOz and 10 ppm Hz, 14 ppm Nz, 14 ppm Ne, and 12 ppm HzO. To purify the helium carrier gas, we used a hydrox purifier (Matheson, LaPort, TX) which contains a zirconium, titanium, and nickel alloy. This alloy when heated is effective in retaining some of these impurities, but others such as Hz,Ne, Ar,and Nz just pass through. The HzO content in helium is reduced to Hz increasing the content of Hzto twice that of HzO. To further purify this helium grade, we inserted a well-conditioned molecular sieve trap upstream from the hydrox purifier. The molecular sieve trap will effectively retain most of the HzO and COP With this arrangement all the high-purity helium cylinders produced a partial negative response to Hz, Ar + Oz, and Nz. After a partiial negative respoinse to Hz, Ar + 02, and Nz was achieved, gaseous additive was added to the helium
to achieve the minimum background current. The addition of gaseous additives was carried out via the permeation tube technique (9). Hz, Ar, 02, or N2 could be used as gaseous additives (3-5); however, we used Hz in the work described here. Table I1 shows the magnitude of the detector response at the minimum background current using the two grades of helium. The variation in the detector response, when different cylinders of helium are used, is now minimized. It appears from Table I that for most applications the high-purity grade of helium is adequate and provides a positive response for almost all gases and compounds tested. Ultrahigh-purity helium shows an improved response, but it provides a negative response to some selected gases. The variation in the detector response with different cylinders of helium could be minimized by adjusting the total impurities to that which produces the minimum background current.
LITERATURE CITED (1) Bourke, P. J.; Dawson, R. W.; Denton, W. H. J . Chromatogr. 1984, 14, 1387. (2) Bourke, P. J.; Gray, M. D.; Denton, W. H. J. Chromatogr. 1985, 19, 189. (3) Bros, E.;Lasa, J. J . Chromatogr. 1979, 174, 258. (4) Andrawes, F.; Gibson, E. Anal. Chem. 1978, 50, 1146. ( 5 ) Andrawes, F.; Gibson, E. Anal. Chem. 1980, 52, 1846. (6) Andrawes, F.; Brazell, R. S.; Gibson, E. Anal. Chem. 1980, 52, 89. (7) Andrawes, F.; Gibson, E.; Bafus, D. Anal. Chern. W80, 52. 1377. (8) Andrawes, F.; Eyers, T. B.; Gibson, E. K. J . Chromatogr. 1981, 205, 419. (9) O'Keeffe, A. E.; Ortman, G. C. Anal. Chem. 1988, 38, 7CO.
RECEIVED for review April 17, 1980. Resubmitted January 5,1981. Accepted March 17,1981. This work was performed, in part, under the auspices of the National Aeronautics and Space Administration, Contract NAS9-15800 to Lockheed.
Cooling System for High-Power Ion Lasers S. W. Edwards and J. F. Schetzlna" Department of Physics, North Carolina State University, Raleigh, North Carolina 27650
Most commercial ion lasers are equipped with water cooling systems which require flow rates of 3-8 gal/min. In principle, ordinary tap water can be used in these systems. However, the use of poor quality w,ater can lead to several detrimental effects. Particulate matter in the water can block water passages in the laser power supply and plarima tube and thus increase the system's flow impedance. This problem can be reduced but not eliminated by using filtered tap water. Water having an appreciable ion content is also undesirable as a cooling medium. Magnes#iumand calcium ions, for example, can precipitate out to form scale on the walls of the laser water passages thereby decreasing the efficiency of the cooling system. The Spectra-Physics line of ion lasers, in particular, should be cooled with water which is sufficiently purified so that water in a 10 ft length of 0.5 in, diameter hose has at least 100 kQ resistance when measured under flowing conditions ( I ) . This requirement follows from the plasma tube design employed by Spectra-Physics in which the plasma tube anode passes through the water cooling jacket. Thus, the cooling water is in direct contact with the anode and can be considered to be at that potential. High conductivity water in the cooling system, therefore, provides an alternate path to ground. In extreme cases, this could lead to unstable laser output power or premature plasma tube failure.
This paper describes a simple recirculating loop cooling system designed to eliminate the above potential problems. The system employs off-the-shelf plumbing components where possible to minimize cost without sacrificing performance. The system is most appropriate for small industrial or university laboratories where the cost of commercial cooling systems of similar design may be prohibitive.
DESCRIPTION OF COOLING SYSTEM The major components of the cooling system are shown schematically in Figure 1. The pump, laser, heat exchanger, and reservoir form a recirculating cooling loop which employs deionized water. The water purity is maintained by a deionizer located in a parallel line between the pump and the reservoir inlet. The pump is a Rurkes Model 3CT6M close coupled turbine pump (Burkes Pumps, Decatur, IL) of all bronze construction which provides a water flow rate of 4.3 gal/min at a pressure of 70 psi. The water from the laser is cooled with an AmericanStandard Model BCF-06-036-001 shell and tube type heat exchanger (American Standard, Buffalo, NY). This particular model is a single pass unit having a diameter of 6 in. and a length of 36 in. The heat exchanger is used in a counterflow configuration as shown in Figure 1 in which water in the
0003-2700/81/0353-1545$01.25/00 1981 Amerlcan Chemical Society
1540
Anal. Chem. 1981. 53, 1546-1548
/
I
C
1
\ SE
Figure 1. Schematic of cooling system: (A) reservoir vent; (B) reservoir fill cap; (D) deionizer; (F) water filter; (M) water flow meter; (P) water pressure gauge; (T) water temperature gauge.
recirculating loop passes through the shell side of the unit. Filtered tap water flowing through the tube side of the heat exchanger removes the heat generated by the laser. The reservoir is constructed entirely of schedule 40 poly(vinyl chloride) (PVC) material which is readily obtainable from local plumbing supply houses. It consists of a 4 ft length of 6 in. diameter PVC tubing, two 6 X 6 X 1in PVC tees, and two 6 in. PVC end caps which are bonded together with heavy duty industrial grade PVC solvent cement. The reservoir has a capacity of about 35 L and is normally filled with 25 L of deionized water during use. The water lines from the heat exchanger to the reservoir and from the reservoir to the turbine pump, which carry cool water, are made from 1 in. diameter PVC tubing. Hard copper tubing (3/4 in. diameter) is used between the laser and the heat exchanger. The cooling system contains two water filters, two water flow meters, two pressure gauges, and four temperature gauges arranged as shown in Figure 1. Ameteck Plymouth Supreme Model PS-S1water filters (Ameteck, Plymouth Products Div., Sheboygan, WI) with type S1 replaceable cartridges are used to remove particulate matter from the water in the recirculating loop and from the tap water flowing through the heat exchanger. Flow rates are measured with Hershey-American Model 430 water meters. These flow meters are of bronze construction and are obtainable from local plumbing suppliers. The differential water pressure across the 171 laser system is measured with two Ashcroft 0-100 psi dial type pressure gauges. Water temperatures a t the inlets and outlets of the heat exchanger are monitored with Ashcroft 10-149 "C bimetal thermometers.
The deionizer consists of a series arrangement of two Barnstead type HN series 8900 mixed bed ion removal cartridges. When the cooling system is in use, the valve preceding the deionizer is adjusted to obtain the desired water pressure at the laser inlet (approximately 65 psi). This allows a fraction of the recirculating water to continuously pass through the deionizer thereby maintaining its purity.
COOLING SYSTEM PERFORMANCE The above cooling system has been used for the past 3 years to cool a Spectra-Physics 171-18 argon ion laser with excellent results. In operation, a flow rate of 4.2 gal/min is maintained in the recirculating loop, as is recommended by SpectraPhysics. To adequately cool the laser system when it is operating under full output power, a minimum tap water flow rate of 6 gal/min at 23 "C is required. In the summer months, when the tap water reaches a maximum of 30 "C, a flow rate of 8 gallmin is employed. These are modest requirements which are ordinarily satisfied by standard laboratory plumbing facilities. With the laser operating at full output power (-50 A tube current) the cooling system extracts approximately 34 kW of heat from the system with an efficiency (2)greater than 0.5. No problems with the cooling system have been encountered during the 3-year period it has been employed. The laser plasma tube has remained in a near virgin state with no visible evidence of scale buildup or other water contaminants. The Burkes turbine pump has performed flawlessly while providing a very stable water flow rate through the recirculating loop. Although complete cooling systems of similar design are available from commercial vendors, the present system can be assembled in a matter of hours and at a substantial savings. The total cost of the cooling system described here is less than two percent of the cost of the Model 171-18 ion laser and its performance to date suggests that it is well worth the time and effort needed to construct it. LITERATURE CITED Laser Instructlon Manual (C/171 9/ 77); Spectra-Physics, Sunnyvale, CA; p 4-3. Simonson, John R. "An Introduction to Engineering Heat Transfer";
(1) Spectra-Physics Model 171 Ion (2)
McGraw-Hill: New York, 1969; Chapter 2.
RECEIVED for review February 9, 1981. Accepted April 27, 1981. Work supported in part by NSF Grants DMR77-08443 and DMR79-11336.
Silanized Carbon Microneedle Emitters for ChemicaVField Ionization David F. Fraley and Maurice M. Bursey" Wililam Rand Kenan, Jr., Laboratories of Chemistry, The University of North Carolina, Chapel Hiil, North Carolina 27514
I t is now possible to attach surface functional groups covalently to metal emitters used in some laboratories for field desorption (FD) mass spectrometry ( I , 2). These groups are not removed by the combined field, vacuum, and emitter heating used in FD. Further, the influence of the field on the acidity of attached carboxylic acid groups is profound: the carboxylic acid, in the strong electric field used in FD and field ionization (FI),is strong enough to protonate representative olefins, alcohols, and even alkyl halides. Because in FD untreated emitters may produce either M+. or (M f l)",this
protonating ability of the modified acid surface emitter is particularly useful. In the field the heated carboxylic acid emitter cleanly ionizes all molecules with functional groups as (M 1)". Further, although other methods for forming (M f I)+ions exclusively have been found, they involve the use of acids which are strong even in the absence of an electric field coating a surface with polyphosphoric acid (3) or mixing the sample with p-toluenesulfonic acid ( 4 ) runs the risk of acid-catalyzed decomposition of an especially fragile molecule to be analyzed before the ions can be obtained. In the absence
+
0003-2700/81/0353-1546$01.25/00 1981 American Chemical Society
