John R. Moody National Bureau of Standards Washington, D.C. 20234
NBS CleanLaboratories
for TraceElement Analysis The need for reliable and accurate measurements of elements a t trace and ultratrace concentrations is now well established. Less well known is the fact that often the key to successful analysis of a sample is found to be in the control of the analytical blank. As shown by Murphy (I),the analytical clean room used to produce a particulate-free environment is one of the major tools available to the analyst. The potential for trace element contamination from the air has been examined by Murphy (I),Patterson and Settle ( 2 ) ,Zief and Mitchell ( 3 ) ,Zief and Nesher ( 4 ) ,and many others. Such particulate contamination may be controlled by the high-efficiency particulate (HEPA) filter ( 5 ) , which was developed during World War I1 for the Manhattan Project (6) and was used to provide absolute containment of radioactive particulates within the laboratory. Today, the same application of HEPA filters is used in radioactivity laboratories everywhere. HEPA filters may have a minimum efficiency as great as 99.99% for 0.3-wm particles. Filters of this type were used very early in laboratories developing the transistor and subsequent solid-state devices and in microscopy laboratories. Clean benches were commercially available in the sixties, and a laminar flow exhaust hood was described at that time (7,8). The Federal Standard 209b ( 9 ) ,which defines the Class 100 specification for measurement of particulate air quality, was developed to assure reliable 1358A
procurements for federal agencies and also dates back to the early sixties (10).
Geologists and geochemists seem to have been among the first, along with semiconductor manufacturers, to have a need for low contamination in ultratrace analysis or production. Prior to 1971, Patterson and colleagues at the California Institute of Technology (CIT) and Tatsumoto of the U S . Geological Survey, Denver, had achieved low P b blanks in their work. Patterson and Settle have described the CIT clean room operation in great detail ( 2 ) .Mitchell described a chemical clean laboratory in ANALYTICAL CHEMISTRYin 1973 (1I ) . Zief and Nesher described an approach to clean laboratory design in 1974 which approached Class 100 conditions ( 4 ) . When the National Bureau of Standards (NBS) undertook to build a clean laboratory in 1971, there were no complete laboratories designed specifically for chemistry that met Class 100 specifications, with the possible exception of Patterson’s laboratory. A handful of chemists performed trace element chemistry in what have been described as “dust-free” or “particlefree” rooms designed for low contamination. At the same time, NBS already had two large clean rooms, but these had proven unsuitable for ultratrace analytical applications. The specific need was to build a laboratory capable of accommodating several chemists analyzing the Apollo 14 lunar rock and soil samples for dif-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
ferent elements (12).At that time commercially built clean rooms could not meet requirements for trace element chemistry. The NBS staff designed a new laboratory from the floor up and contracted with an outside consultant for the construction and testing of the lab. At this point a distinction should be made between a clean room and a clean laboratory. A clean room is designed to maintain Class 100 air quality preferably with a minimum of activity in the room. NBS applications required a facility with few impediments to the chemists’ use of the lab, but with a controlled clean air flow that would make it possible to attain exceedingly low analytical blanks. Thus, the clean laboratory permitted all wet-chemical operations that were difficult to perform in a clean room. Since 1971, new laboratories have been built and numerous improvements have been made. The following discussion is confined to NBS work. Nevertheless, it is hoped that this REPORT is complete enough to help the reader select key factors for the design of clean laboratories.
Traditional Class 100 Requirements Though many chemists question the need for a clean room or lab, the environmentally induced blank comes largely from the particulate matter in the laboratory air. Murphy (1)has This article not subject to U.S. Copyright Published 1982 American Chemical Society
b
View from rear of one section of clean lab. The emergency exit is in the background and the Teflon fume hoods are on the left
0.3 1.0
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Paaicie Size (pn) Flgure 1. Permissible particle size dis-
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are unreliable except when a b a r g enurnbw of ssmpllngs Is taken. Figure adapted from Federal Standard 209b
presented a concise description of the origin and control of the analytical blank. Both the elemental composition and numbers of particles may be expected to vary with location. Gaithersburg, Maryland, the site of
the NBS analytical clean lab, is a suhurban location in a city free of heavy industry. However, the NBS laboratories are immediately adjacent to a busy intemtate highway, and there is some farming near Gaithersburg. Thus, P h particulate concentration becomes a function of the time of day, while calcium and some other elements have a seasonal variation associated with tilling, fertilizing, and other farm or residential activities. Particulate counts above 0.3 pm in NBS labs average between 26 OOO44 OOO particlesL. (750 W 1 2 5 0 OOO particles/ft3). The Federal Standard 209h (9) defines a number of different air qualities, with the Class 100 standard being the most stringent. Although the requirement is frequently stated as a measurement of no more than 100 particles per ft3 which are 0.5 pm in diameter or larger, it is actually a bit more complicated. The important analytical consideration is the total mass of particles, regardless of their size or number. From Figure 1it can be seen that the restrictions imposed by the standard are a function of particle size. and the standard is considerably more ytrict for larger particles. Fortunately, the efficiency of the HEPA filter is at a minimum at -0.3 cun rather than between 1-10 pm, where the particles that contribute most to the analytical blank are found. Very small particles do not have the mass of larger ones and thus contribute less t o a blank problem. The influ-
ence of mass with greater particle size is reflected in the Federal Standard 209b requirement. The management of process-related particles under turbulent air flow is difficult. Thus, the advantages of laminar flow were also recognized at an early date (7).and nearly all clean room designs have used laminar or nonturbulent air flow designs. The 209b standard also describes designs for complete laminar flow rooms, clean benches, and fume hoods. The standard contains a wealth of information on design, testing, and maintenance of clean rooms, and should be considered an essential reference for those interested in a clean laboratory. Meeting Class 100 requirements alone, however, has not proven to he a guarantee in meeting specific goals for contamination control relevant to trace metal analysis. This is because the composition of the contaminant can be more important than the quantity. For example, in a trace P b analysis, a single 0.5pm particle of lead would pose a far more serious problem than a 10-pm particle of typical soil composition. Within the chemistry laboratorv. the wear narticles and corrosion paiticles become a problem potentially more severe than the normal air particulate. Thus, to control the analytical blank for trace metal chemistry, one must control the total air quality as well as the composition and type of materials used to build the laboratory. To the extent that this latter requirement is not practicable, the
ANALYTICAL CHEMISTRY, VOL. 54. NO 13. NOVEMBER 1982
1959I
laboratory design should be adjusted to compensate for such compromises. Evolution of the NBS Design Prior to 1971, the NBS experience with clean rooms had been with the horizontal laminar flow type depicted in Figure 2a. This laboratory easily met Class 100 requirements and continued to do so for many years. Originally the laboratory bad been built for the handling of samples for surface analysis as well as for conventional optical microscopy. At the same time, a number of conventional horizontal as well as vertical flow clean benches were available. The main problem with the type of clean room shown in Figure 2a was that of cross contamination caused by the flow of air from one work surface to another. Very careful and not always successful coordination was required for more than one chemist to use the laboratory at the same time. Thus, even if contamination levels were successfully controlled, full utilization of laboratory space was difficult. One solution to this would have been to build a laboratory similar to the design shown in Figure 2b. In a totally vertical air flow system, cross contamination would be virtually impossible, since the air is exhausted downward immediately after passing the work surface. Every work surface is exposed to the same quality of laminar-flow clean air. There are, however, a number of problems related to the pure vertical flow system. First, this design is quite expensive to build, requiring a large number of air filters and commensurately higher energy costs. Second, use of the false floor and a drop ceiling requires a total clearance for construction that can be troublesome. Finally, even when cost alone is not a constraint, care must be exercised in the choice of construction materials. For example, although perforated metal flooring would be acceptable for a computer clean room, i t would present obvious disadvantages in a trace element chemistry laboratory. Because of the initial cost requirement for the pure vertical flow laboratory, no serious attempts were made to find or design an improved floor material for the false floor of this design. The final result was a hybrid design combining the favorable features of the previous designs. The design is shown in Figure 2c and was achieved in consultation with the contractor for the first NBS clean lab. The design has numerous advantages uniquely tailored for the trace analytical laboratory. First, a supply of Class 100 air is delivered at the primary work snr1360A
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ANALYTICAL CHEMISTRY, VOL. 54. NO. 13. NOVEMBER 1982
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face, the center bench of the laboratory. Since all corrosion-prone air diffusion screens were eliminated, nothing stands between the supply of filtered air a t the HEPA filter and the analytical sample at the work surface. Next the air travels across the bench towards the aisles, which isolates the sample from the chemist, who is one of the major sources of contamination (I). Lastly, the air moves over the floor and secondary work benches on the side walls. These benches are reserved for peripheral equipment such as balances, ovens, spectrometers, and centrifuges. Thus, these pieces of equipment, considered unavoidable major sources of contamination, are placed immediately next to the air exhaust and farthest from the samples. I t cannot be overemphasized 1362A
that contaminants must he effectively removed at the source to minimize the spread of these contaminants in the clean lab. The NBS design (Figure 2c) is as effective as the pure vertical flow design (Figure 2h) in particulate removal. Yet, the design is relatively uncomvlicated and comparable in cost to the horizontal flow lab (Figure 2a). Conventional- and laminar-flow clean air exhaust hoods are located on the side benches. A section of the air return wall is replaced by exhaust ducts for the fume hoods. Since the air flow required for a fume hood is comparable to that handled by a section of air return wall, the addition of one or more fume hoods along the side walls causes no disturbance in the pattern of air flow in the laboratory.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
Most of the air in the laboratory is recirculated. However, since the air through a fume hood is totally exhausted, sufficient make-up air must be introduced above the false ceiling to compensate for the amount of air being exhausted. Operating laboratories at NBS have exhausted-to-circulating air ratios ranging from 1:3 to 1:6 depending upon the number of fume hoods in use. Because the normal room air change per hour requirements for most chemistry laboratories are substantial, relatively little difficulty should be encountered in achieving an exhausted-to-recirculated air ratio of 1:6. Obviously, the greater the amount of exhausted air, the shorter will he the residence time of unwanted fumes in the laboratory. The original performance objectives
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were achieved in the present design, which permits the use of equipment that could not have been tolerated in a horizontal-flow laboratory. The vertical air flow and short direct exhaust air routes assure that cross contamination between samples isl virtually eliminated. Contamination cannot be carried from point to point around the laboratory unless physically moved by a chemist. The need for a clothing change room has been eliminated; indeed, one can walk into the laboratory in street clothes with no demonstrable effect on ambient air quality in sample work spaces. To minimize the entry of dirt into the lab from chemists’ feet, a set of sticky mats is kept at the entry door, where covers are placed over shoes before entering. Inside, a clean room garment and cap are hung on hangers or stored in drawers for wear after entry to the room. The entire entry or exit procedure takes less than 1 min. This feature has been an important one in gaining acceptance of the laboratory by the many chemists using the facility, each of whom may enter and exit up to 10-20 times per day. A concession to the individual chemist’s need for defined space was the placement of a series of Plexiglas dividers on the center clean bench. This permits a chemist to use a particular space, and reduces accidental incursions into unassigned space, which might lead to confusion or sample cross contamination. The layout of the laboratory (Figure 3) i s quite flexible and is easily expanded in width or length by combinations of the basic air flow pattern. We feel this design has forced the available clean air to do the maximum amount of work and at the lowest initial cost of all of the alhrnative approaches explored. While the air flow pattern is highly favorable for a chemistry laboratory, there are many other details to the design that are essential for obtaining a working laboratory with low trace element blanks. The following section briefly details some of these requirements. The intention is to provide an outline of what to look for, while developing readers’ sensitivities to potential problems they may encounter in their own laboratories. Specific evaluation criteria may be quite different from those expressed here, and this will be byiefly discussed later for the example of a trace organic laboratory. Unique Requirements for Trace Element Chemistry Regardless of the particular instrumental technique used for measurement, certain basic chemical processing steps must be performed initially in trace element analyses. These may
include sampling, storage, drying or reconstitution, chemical dissolution, and perhaps elemental separation by ion exchange, extraction, or chelation, to name just a few. For each step of the procedure, contamination of the sample may occur from the sample container, apparatus, the chemist, reagents, and the environment ( I ) . T h e sole purpose of the analytical clean lab is to control the environmental chemical blank, which may be airborne or laboratory-induced. The Class 100 specifications (9) are meant to define particulate loading in the air of a work zone, but this does not necessarily correlate with the measured chemical blank. The problem is that the chemist intends to produce a blank below a given level, but cleanroom specifications only define particulate concentrations. Thus, in one laboratory designed to exclude P b entirely, Class 100 000 air might produce acceptable blanks for P b while another Class 100 laboratory with little attention paid to construction materials might produce an unacceptably high P b blank. Thus Class 100 specifications are an inadequate measure of laboratory performance if the chemical blank is the performance criterion. The best assurance continues to be to rely on the techniques, design, and construction used by those laboratories that have proven low laboratory blanks, and it is t o that end that this paper was prepared. Class 100 specifications form a useful basis for beginning to measure air quality, but the chemist must still devise a means of measuring the laboratory blank for the element of interest. This blank will be a function both of air quality and the nature of construction of the laboratory. The blank problem was approached from the point of view of design by elimination of contaminants. For example, sinks and plumbing traps could not have P b parts and Cu pipes and solder joints had to be painted or covered to effectively place them outside of the laboratory environment. Much of our education in materials durability took place in the first several years of operation of the laboratory. Despite problems, the laboratory has always performed as well as expected and has proven to be superior in blank levels to the conventional clean room. The first five years of use led to changes that were incorporated into the designs presented here. From the beginning, the NBS laboratory design incorporated the fact that many foreign objects and pieces of equipment are normal and necessary to the operation of a chemistry laboratory. Furthermore, simultaneous multielement contamination
control was of paramount importance when the laboratory was occupied by a number of chemists working on different sample matrices and on the determination of different elements. Thus, the second function of a clean room or lab, to exhaust and remove particulate matter as quickly as it is generated, became one of the primary design goals. This is even more important when it is realized that a single chemist may be in and out of the laboratory many times per day, since some operations such as sample dissolution do not require full-time observation. Based on our experience with commercial clean rooms and with the NBS-type clean laboratory, the following suggestions are offered to help others in determining their requirements. Class 100 requirements are necessary, but virtually every producer or supplier of equipment will meet this requirement. One must exclude the elements of analytical interest from materials of construction and laboratory apparatus, although an occasional compromise may be necessary when alternative materials or apparatus are not available. Insist on durable materials and test them or consult with successful operating laboratories. Failure to do so may lead to unacceptable laboratory conditions in all too short a time. The risk from corrosion in a clean lab or room is actually many times greater than in a conventional lab. Whereas an ordinary lab might have 10 room air changes/h with drafts insufficient to transport heavy particles, the recirculation of air within our clean lab is a t the rate of once every 30 s, and the velocities are sufficient to transport heavy particles a considerable distance. Thus, depending upon the source of contamination, a corroding clean room can become a much worse offender than a conventional lab, since increased air flows increase the risk of undesired contamination transport to the sample. The paradox presented by the prospect of increased risk of contamination due to use of the clean lab is avoided only when the laboratory is built to eliminate the possibility of such contamination. This requires a combination of proper materials selection, corrosion resistance, and materials durability. Safety Requirements An efficient, reliable exhaust system is absolutely essential for proper fume control in the clean laboratory. Longterm effectiveness of the clean laboratory is thereby assured. At NBS, the laboratories have a high ratio of exhaust to recirculating air (as great as 1:3), which assists in maintaining low fume levels in the laboratory even when using 8 M HC1 for anion exchange chromatography.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
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For inorganic analysis, all ductwork from a clean lab should be constructed from polyvinyl chloride (PVC) or some other inert plastic. Airflows within the clean laboratory will be constant over an indefinite period of time. Galvanized duct-work, however, gradually develops leakage due to corrosion, which leads to less efficient exhaust and imbalanced airflows. Ordinarily this is not too serious until the fume hood performance is impaired. However, when this happens with a laminar-flow exhaust hood, some fumes will be introduced into the laboratory as the airflows become unbalanced. It is possible to adjust the laminar airflow, but the difficulty lies in the delay between the onset of a problem and its detection. During this time a considerable , amount of acid fumes can be recirculated in the laboratory, causing damage to metal parts such as motor and fan bearings and possibly creating unhealthy conditions. Of course, perchloric acid should never be vented through metal ductwork. The best solution to the problem is to employ PVC ductwork, which is impervious to acid attack. To avoid problems, solvent-welded joints and heat-welded joints should be avoided when they come in direct contact with the exhaust air stream. These joints are mechanically weak and will eventually yield to continual acid attack. The Laboratory of the Government Chemist in London has published appropriate standards that should be considered for future laboratory duct construction in the U.S. (13). Two levels of fire detection are provided. Smoke detectors are located in the false ceiling just above the aisles in the laboratory and in the recirculating air stream. Thus, a fire can be detected much more quickly than with the conventional rate-of-rise heat detectors that are installed in the original laboratory ceiling as a back-up system. The locations of the smoke detectors were chosen to be the least objectionable while still satisfying functional requirements. Another compromise in the laboratory design was the inclusion of a metal eyewash fixture (none could be found made of plastic) at the end of a retractable hose which was installed at the sink. Although the fixture could corrode, it provides an instant stream of water directable to the eye or other parts of the body. A traditi6nal safety shower is installed just outside the entrance to the laboratory. Doors to the hallway are in line and easily opened in the event of an emergency. Most laboratories have some problems with the safe storage of solvents and acids, and the clean lab is no different. In particular, provisions must 1366A
be made for the separate storage of solvents and oxidizing acids. A polypropylene fume hood is used to store acids and to vent acid fumes. The Teflon bottles commonly used for ultrapure acids breathe significant amounts of acid vapor into the air, and these must be properly vented. Limited quantities of solvents are permitted in designated base cabinets in the clean lab. The usual metal safety cabinet and containers are not compatible with trace element work. Thus, extreme caution is needed when solvents are used, and this was yet another reason for improved fire detection by means of the smoke detectors. Clean Laboratory Component Requirements HEPA Filters. HEPA filters are manufactured by a number of commercial sources in the U.S., including Cambridge Filters, Flanders Filters, Mine Safety Apparatus, and others. The efficiency of the filters varies with the intended application. The better grades are 99.97% efficient at 0.5 pm, and are individually probe-tested for leaks. This quality of filter, which may be called laminar-flow grade, is the only type that should be used in a trace element laboratory. Although the efficiency of the filter is one of the most important parameters to be considered, other factors are equally important. The ultimate effect of low-level corrosive fumes on the filter must be considered if long-term laboratory performance is to match original performance levels. To our knowledge, a plastic framed filter is not commercially available. Our experience has shown that the filter itself can become the source of particulates, the composition of which are more damaging than the original unfiltered laboratory air. Rust particles, for example, are a more serious problem than a similar amount of silicate-based particulates. Thus, wood, plywood, or particleboard frames are required in place of a metal frame that would eventually corrode. Most HEPA filters contain pleated strips of aluminum foil between the folds of the HEPA filter medium, which provides additional support to the filter medium. Although aluminum metal has little tendency to corrode compared to stainless steel, it can act as a site for the deposition of NH4C1, which eventually falls on the bench below. The best solution to the problem is to avoid aluminum separatortype filters. Several commercially available alternatives are possible, including an HEPA filter without a separator or with paper separators. Both of these
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
filters have been extensively used at NBS with success. Also available is an HEPA filter utilizing a PVC separator. Since all of these products are readily available, the lab designer must make a decision on original cost vs. estimated useful life. At NBS we concluded that the slight additional cost for longer-lasting filters was negligible compared to the cost of lost laboratory time required for premature filter replacement. The silicone and foam sealants used to secure the HEPA filter material to its frame have shown excellent chemical resistance. An equally important seal is the one made between the HEPA filter frame and the air plenum into which the filter is secured. This seal is usually made by a compressible gasket between the mating surfaces of the filter and the plenum. Since the anticipated life of the filters is at least 5-10 years, the durability of this seal is a major concern. Any leakage would introduce unfiltered air into the filtered air stream. Compressible gaskets of polyurethane foam, rubber, etc. have been found to have a short lifetime in some laboratory environments. This failure causes the need for replacement of the filter long before its useful lifetime. For this reason, the “greasy groove” seal used on some filters is preferred. This system uses a routed groove in the filter frame filled with a nonhardening silicone grease. It mates against a flange on the filter plenum, which sinks well into the groove of the filter. The resultant seal has been found to be stable to chemical attack for 10 years. None of the “greasy groove” filters has been removed from service due to gasket leakage or filter clogging. The silicone does not take a “set,” so that some unintentional distortion in the filter frame or air plenum does not lead to air leakage. The ultimate lifetime of an HEPA filter is a function of loading with particles. In turn, this loading is a function of the quality of the air being filtered. As dirt builds up in the filter, filter efficiency is not decreased and may even be improved; however, a greater pressure is required to force air through the filter. Many, if not most, clean room installations have pressure manometers designed to show the pressure drop across the HEPA filter and/or even the prefilter, which frequently consists of a disposable polyurethane foam. When the pressure drop reaches a prescribed limit, filter replacement is required. Usually the replacement is made to maintain filtered air velocity at a desired level, since the blower fans are usually belt driven and not easily adjusted to compensate for reduced air flow through the filter once it has
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