NBS Clean Laboratories for Trace Element Analysis - Analytical

Report John R. Moody National Bureau of Standards Washington, D.C. 20234

NBS Clean Laboratories for Trace Element Analysis The need for reliable and accurate measurements of elements at trace and ultratrace concentrations is now well established. Less well known is the fact that often the key to success ful analysis of a sample is found to be in the control of the analytical blank. As shown by Murphy (1), the analyti cal clean room used to produce a particulate-free environment is one of the major tools available to the analyst. The potential for trace element con tamination from the air has been ex amined by Murphy (1), Patterson and Settle (2), Zief and Mitchell (3), Zief and Nesher (4), and many others. Such particulate contamination may be controlled by the high-effi ciency particulate (HEPA) filter (5), which was developed during World War II for the Manhattan Project (6) and was used to provide absolute con tainment of radioactive particulates within the laboratory. Today, the same application of HEPA filters is used in radioactivity laboratories ev erywhere. HEPA filters may have a minimum efficiency as great as 99.99% for 0.3-μηι particles. Filters of this type were used very early in laborato ries developing the transistor and sub sequent solid-state devices and in mi croscopy 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 quali ty, was developed to assure reliable

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. Geo logical Survey, Denver, had achieved low Pb 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 CHEMISTRY in 1973 (11). Zief and Nesher described an approach to clean laboratory design in 1974 which ap proached Class 100 conditions (4). When the National Bureau of Stan dards (NBS) undertook to build a clean laboratory in 1971, there were no complete laboratories designed specif ically for chemistry that met Class 100 specifications, with the possible ex ception 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 contami nation. At the same time, NBS al ready had two large clean rooms, but these had proven unsuitable for ultratrace analytical applications. The specific need was to build a lab oratory capable of accommodating several chemists analyzing the Apollo 14 lunar rock and soil samples for dif

1358 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982

ferent elements (12). At that time commercially built clean rooms could not meet requirements for trace ele ment chemistry. The NBS staff de signed 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 de signed to maintain Class 100 air quali ty preferably with a minimum of ac tivity in the room. NBS applications required a facility with few impedi ments 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 improve ments 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 labora tories. Traditional Class 100 Requirements Though many chemists question the need for a clean room or lab, the envi ronmentally 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

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

I II

II 0.5 1.0

5

10

Particle Size (^m) Figure 1. Permissible particle size dis tribution curves Counts below 10 particles/ft3 (0.35 particles/L) are unreliable except when a large number of samplings is taken. Figure adapted from Federal Standard 209b

presented a concise description of the origin and control of the analytical blank. Both the elemental composi tion and numbers of particles may be expected to vary with location. Gaithersburg, Maryland, the site of

the NBS analytical clean lab, is a sub urban location in a city free of heavy industry. However, the NBS laborato ries are immediately adjacent to a busy interstate highway, and there is some farming near Gaithersburg. Thus, P b particulate concentration becomes a function of the time of day, while calcium and some other ele ments have a seasonal variation asso ciated with tilling, fertilizing, and other farm or residential activities. Particulate counts above 0.3 μπι in NBS labs average between 26 00044 000 particles/L (750 000-1 250 000 particles/ft 3 ). The Federal Standard 209b (9) de fines a number of different air quali ties, with the Class 100 standard being the most stringent. Although the re quirement is frequently stated as a measurement of no more than 100 particles per ft3 which are 0.5 μπι in diameter or larger, it is actually a bit more complicated. The important an alytical consideration is the total mass of particles, regardless of their size or number. From Figure 1 it can be seen that the restrictions imposed by the standard are a function of particle size, and the standard is considerably more strict for larger particles. Fortunately, the efficiency of the HEPA filter is at a minimum at ~0.3 μια rather than between 1-10 μπι, 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 contrib ute less to 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 lam inar 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 infor mation on design, testing, and mainte nance 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 be 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 quan tity. For example, in a trace P b analy sis, a single 0.5-μιη particle of lead would pose a far more serious problem than a 10-μπι particle of typical soil composition. Within the chemistry laboratory, the wear particles and cor rosion particles become a problem po tentially more severe than the normal air particulate. Thus, to control the analytical blank for trace metal chem istry, 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 lat ter requirement is not practicable, the

ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 · 1359 A

laboratory design should be adjusted to compensate for such compromises. -Blowers

-Drop-Style False Ceiling

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 had 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, it 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 sur-

-HEPA Filter Bank

-Work Bench

False Wall

-HEPA Filter Bank

- Blowers

Return Air Plenum False Floor -

-Work Bench

False Wall

HEPA Filters

-Modular Blower Filter Assembly

Return Air "Drop" False Ceiling -Plexiglas or Glass Canopy

Optional Class 100 or other Exhaust Hood 4 _

False Wall

Storage Cabinets

Class 100 Work Bench

AuxiliaryBench

Figure 2. Cross-sectional views of clean rooms (a) Horizontal-flow clean room, (b) Vertical-flow clean room, (c) NBS clean laboratory. Reprinted with permission from Phil. Trans. Royal Soc. (London)


Adjoining Laboratories Exhaust Duct Chaseway

Exhaust Duct Chaseway

Polypropylene Exhaust Hood Exhaust Duct

Clean Air Bench

Polypropylene Sink and Drainboard

Utility Space

Polypropylene Sink and Drainboard

Clean Air Bench

Wood Stools (As Needed) Utility Space Explosion-Proof Refriaerator,

Ι Balance Table Emergency Exit Vacuum Hose

Uirty

View Glass

Main Entry

View Glass

Emergency Exit

Utility Service Closet

Double Door

Oven

Sticky Mat

'

-Uniforms Utility Service Closet

Double Door

Utility Service Closet

Central Vacuum Unit Air Return Wall

Short-Term Storage

Hallway New Wall Clad over Existing Wall

Added Partition Wall

I

Utility Service Closet Safety Shower

Existing Laboratory Wall

,

_

Figure 3. NBS Center for Analytical Chemistry trace element clean laboratory (a) Class 100 vertical laminar flow exhausted fume hoods, (b) Class 100 laminar-flow bench for distilled water, (c) Fresh air-conditioned make-up air introduced above entry doors, (d) PVC ductwork for fume hoods

face, the center bench of the laborato ry. Since all corrosion-prone air diffu sion screens were eliminated, nothing stands between the supply of filtered air at the HEPA filter and the analyti cal sample at the work surface. Next the air travels across the bench towards the aisles, which iso lates the sample from the chemist, who is one of the major sources of con tamination (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 contami nation, are placed immediately next to the air exhaust and farthest from the samples. It cannot be overemphasized

that contaminants must be effectively removed at the source to minimize the spread of these contaminants in the clean lab. The NBS design (Figure 2c) is as ef fective as the pure vertical flow design (Figure 2b) in particulate removal. Yet, the design is relatively uncompli cated and comparable in cost to the horizontal flow lab (Figure 2a). Con ventional- 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 re quired for a fume hood is comparable to that handled by a section of air re turn 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.


Most of the air in the laboratory is recirculated. However, since the air through a fume hood is totally ex hausted, sufficient make-up air must be introduced above the false ceiling to compensate for the amount of air being exhausted. Operating laborato ries at NBS have exhausted-to-circu lating 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 require ments for most chemistry laboratories are substantial, relatively little diffi culty should be encountered in achiev ing an exhausted-to-recirculated air ratio of 1:6. Obviously, the greater the amount of exhausted air, the shorter will be the residence time of unwanted fumes in the laboratory. The original performance objectives

I

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 is 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) is 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 alternative 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 briefly 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 (1). The 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 Pb entirely, Class 100 000 air might produce acceptable blanks for Pb while another Class 100 laboratory with little attention paid to construction materials might produce an unacceptably high

Pb 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 to 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 Pb 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 at 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.


For inorganic analysis, all duct work from a clean lab should be con structed from polyvinyl chloride (PVC) or some other inert plastic. Air flows within the clean laboratory will be constant over an indefinite period of time. Galvanized duct-work, how ever, gradually develops leakage due to corrosion, which leads to less effi cient exhaust and imbalanced air flows. Ordinarily this is not too serious until the fume hood performance is impaired. However, when this hap pens with a laminar-flow exhaust hood, some fumes will be introduced into the laboratory as the airflows be come unbalanced. It is possible to ad just the laminar airflow, but the diffi culty lies in the delay between the onset of a problem and its detection. During this time a considerable ( amount of acid fumes can be recircu lated in the laboratory, causing dam age to metal parts such as motor and fan bearings and possibly creating un healthy conditions. Of course, perchlo ric acid should never be vented through metal ductwork. The best so lution to the problem is to employ PVC ductwork, which is impervious to acid attack. To avoid problems, sol vent-welded joints and heat-welded joints should be avoided when they come in direct contact with the ex haust air stream. These joints are me chanically weak and will eventually yield to continual acid attack. The Laboratory of the Government Chem ist in London has published appropri ate standards that should be consid ered for future laboratory duct con struction in the U.S. (13). Two levels of fire detection are pro vided. 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 detect ed much more quickly than with the conventional rate-of-rise heat detec tors 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 objection able while still satisfying functional requirements. Another compromise in the labora tory 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 traditional safety shower is installed just outside the en trance to the laboratory. Doors to the hallway are in line and easily opened in the event of an emergency. Most laboratories have some prob lems with the safe storage of solvents and acids, and the clean lab is no dif ferent. In particular, provisions must

be made for the separate storage of solvents and oxidizing acids. A poly propylene fume hood is used to store acids and to vent acid fumes. The Tef lon 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 per mitted in designated base cabinets in the clean lab. The usual metal safety cabinet and containers are not com patible with trace element work. Thus, extreme caution is needed when sol vents are used, and this was yet anoth er 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 com mercial 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 μπι, 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 con sidered, other factors are equally im portant. The ultimate effect of low-level cor rosive fumes on the filter must be con sidered if long-term laboratory perfor mance is to match original perfor mance levels. To our knowledge, a plastic framed filter is not commer cially available. Our experience has shown that the filter itself can become the source of particulates, the compo sition 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 fil ter medium. Although aluminum metal has little tendency to corrode compared to stainless steel, it can act as a site for the deposition of NH 4 C1, which eventually falls on the bench below. The best solution to the prob lem is to avoid aluminum separatortype filters. Several commercially available al ternatives are possible, including an HEPA filter without a separator or with paper separators. Both of these


filters have been extensively used at NBS with success. Also available is an HEPA filter utilizing a PVC separa tor. 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 neg ligible compared to the cost of lost laboratory time required for prema ture filter replacement. The silicone and foam sealants used to secure the HEPA filter material to its frame have shown excellent chemi cal 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 fil tered air stream. Compressible gaskets of polyurethane foam, rubber, etc. have been found to have a short life time in some laboratory environments. This failure causes the need for re placement 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 dis tortion in the filter frame or air ple num does not lead to air leakage. The ultimate lifetime of an HEPA filter is a function of loading with par ticles. In turn, this loading is a func tion of the quality of the air being fil tered. 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 dispos able 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 de sired level, since the blower fans are usually belt driven and not easily ad justed to compensate for reduced air flow through the filter once it has

View across center bench of main section of clean lab. HEPA filters are near the top of the picture; fume hoods are in the background

"loaded up" with particles. The equipment at NBS incorporates a solid-state variable-speed control to adjust and maintain a desired airflow through the HEPA filter. Once set, these controls do not require readjustment to maintain airflow through the filter, despite changes in environmental factors, filter loading, or other factors. So far, individual filters installed in clean benches with more than 10 years of continuous service have not exceeded the capabilities of the variable speed controller/fan combination.Within one NBS clean laboratory, the same set of filters has been in operation for more than seven years. We are quite optimistic about filter life and believe they can last at least 5-10 years. It is far more likely that a filter will fail due to accidental perforation. The reliability of the equipment used at NBS has been excellent, and no routine maintenance has had to be performed. Out of 30 ceiling modules with approximately 200+ module-years of use, we have experienced a total of one fan and two motor failures, all of which were caused by corrosion. At the present time, the motor bearings appear to be the only weakness for which no remedy is immediately available. Laboratory Flooring

There are a number of approaches

to the choice and maintenance of flooring in a clean laboratory. These comments relate more to NBS experience than to an analysis of all possible alternatives. We have used several approaches with no apparent difference in laboratory performance. However, these results are probably closely related to the intrinsic airflow pattern in the NBS clean laboratory, which minimizes the possibility of introducing dirt from the floor into the air stream of the laboratory. This is achieved by providing an air exhaust at the floor level that both eliminates dead air zones and removes dirt from the floor as it is generated. With proper airflow, the influence of the flooring will be minimal. Other laboratory designs will require a reassessment of our experience based upon performance requirements of those designs. At NBS we have used both coved and noncoved designs as well as seamless and conventional tile flooring. Coved and seamless floorings contain chemical spills effectively, but both designs add considerably to cost. The effectiveness of all flooring types has been equal. The chemical resistance of seamless vinyl or square vinyl asbestos tiles to chemicals is fair to good. One advantage of the conventional tile is that if damaged, it is easily replaced. Of course, any flooring should be consistent with the purpose of the laboratory. In this case, for ex-

ample, a stainless steel door threshold would be unacceptable. None of the laboratory flooring we examined exhibited any signs of particulate production.· Floor maintenance, however, has an impact on laboratory operation. Some unique approaches have been suggested. For example, Patterson (2) recommended the installation of floor drains and the weekly rinsing of the floor with distilled water. The only maintenance given the flooring at NBS is vacuuming with a remotely located central unit, which ideally should be performed daily. Sticky mats with no dirt-trapping frames are used at the entry doors to reduce the migration of dirt into the laboratory. These work reasonably well if fresh layers of the mat are kept exposed. At intervals of approximately three months, the floors are stripped clean and then waxed with a soft wax, e.g., carnauba wax. Modern self-shining acrylic floor finishes are too brittle and produce dust. The appearance of soft waxed flooring deteriorates very quickly as dirt becomes ground into the finish. However, this dirt will remain trapped until it is removed by floor cleaning. One commercially available, but expensive, type of PVC flooring has a washable sticky finish and functions like a wall-to-wall sticky mat. At NBS, we use a low-maintenance, low-cost approach that disturbs the operating routine of the clean laboratory as infrequently as possible. This approach was not planned, but rather evolved as experience was gained in operating the laboratory. Ceiling Construction

All of the clean laboratories at NBS have a relatively light weight suspended ceiling that forms part of the air return plenum. In keeping with the general requirements of materials for trace element analysis laboratories, ceiling materials are selected on the basis of low particulate production, resistance to corrosion, and low trace metal content. Thus, the major metallic components used for the ceiling grid system are fabricated from aluminum. The corrosion resistance of these components is enhanced by heavy spray coatings of polyurethane paint. No failures have been found in seven years of experience with these materials. The ceiling panels are made from particle board with a plastic laminate or finish on each side to prevent warping and particle formation. This material has proven to be noncontaminating and tolerant of acid fumes. The panels are fairly heavy but require no more support than do most suspended ceilings. Laboratory lighting is provid-


View of Pure Reagents Laboratory with a PVC clean air module for distillations to the left and a PVC clean-up station and fume hood to the right ed by standard 0.61 X 1.22 m (2 X 4 ft) fluorescent fixtures for drop-in ceiling installation. With a plastic diffuser panel installed on the bottom, the fixture itself is effectively outside of the laboratory. No effort has been made to design special fixtures since it was considered more cost-effective simply to replace any fixture that became corroded. Thus, all light fixtures have been of relatively conventional aluminum construction with solid plastic light diffusers. In more recent laboratory conversions at NBS, the original light fixtures are left in place, and below the fixtures a light-weight aluminum grid work is used for the false ceiling. Translucent plastic light diffusion panels are installed in the gridwork to complete the ceiling. Since the wall and floor are white or light colored, the final illumination levels in the modified laboratory exceed those in the original laboratory. In addition to saving some construction costs, this design effectively removes the lighting fixture, its copper socket, and its iron ballast transformer from the clean laboratory. The only remaining penetration through the false ceiling is a smoke detector, which is a reasonable compromise in design integrity for safety. Functionally, the ceiling also serves as part of an air plenum for the return of "dirty" laboratory air to the HEPA filter modules. In our first laboratory we were quite concerned about the leakage of air through the seams of the ceiling panels. Accordingly, all of the

panels were sealed in place from the top side using duct tape. Later this practice was changed to using a bead of nonhardening silicone compound between the support frame and the ceiling panels. It should be remembered that below the ceiling the air pressure is positive due to the supply of filtered air while above the ceiling the air pressure will be negative since this area forms an air return plenum. Because of high air pressure within the laboratory, there is no tendency for dirty air to enter the laboratory through the ceiling. Based on NBS experience, a moderate amount of air leakage out of a clean lab is not harmful and laboratory construction can be less than "tight." Since the original laboratory walls and ceiling do form part of the air return plenum, some consideration should be given to sealing these surfaces before construction begins. Excessive corrosion or deterioration will not influence the clean lab air quality, but it may cause premature loading of the HEPA filters. Our steel laboratory walls are painted with polyurethane paint for corrosion resistance, while concrete walls or blocks are sealed with epoxy paint to eliminate concrete dust. Wall Construction

Some of the inside laboratory walls are functional air return walls. These are made by furring out the new wall construction on top of the existing walls; this is why the original laboratory wall should be in good condition. The wall panels are 1.22 X 2.44 m (4 X


8 ft) sheets of particle board laminated with plastic in the same manner previously described for the ceiling panels. Other materials could be substituted with due consideration for trace element content, durability, resistance to particle production, and cost. The panels are aligned vertically and held in place at the edges by an aluminum extension, which in turn is screwed to the furring strips. The top edge is positioned just above the planned ceiling height and ultimately the ceiling supports mount flush to the false wall. The lower edge is brought down to within approximately 10 cm (4 in) of the height of the lab benches. The area from the bench top to the panel is composed of a decorative plastic grill that runs the entire length of the air return wall. Aluminum extrusions are used to retain and edge all joints, corners, and edges. The extrusions themselves are fastened to the furring strips, and a vinyl insert is pushed into a channel to cover up the screws. The result is a smooth wall surface with functional attributes and visual appeal. Where a counter top and lab bench adjoin the air return wall, they form the remainder of the laboratory wall, as shown in Figure 2c. Note that a significant proportion of return air (~one-third) flows beneath the cabinetry and up the wall behind it. The bottom and rear of the cabinetry are sealed for this reason. If no base cabinet is used, additional paneling is extended to the floor, where another continuous plastic grill is installed. At the junction between the ceiling and air return wall a third strip of plastic grill forms the high-level air return. The result is a balanced distribution of return air at the floor, ceiling, and bench levels of the return air wall. This pattern of airflow tends to eliminate dead air zones within the laboratory. The exact size and location of the exhaust air grills must be engineered, since no dampers have been provided to balance airflow. The exposed aluminum extrusions have been very corrosion resistant. After more than six years in the first laboratory, they are still smooth and bright. Under similar circumstances, no coating or paint has been found that prevents corrosion for steel or stainless steel. Other partition walls are made by constructing a grid work of studs or supports and attaching panels to both sides using the same type of aluminum extrusions. One advantage of this design is that the panels are not actually glued, nailed, or screwed and thus they are easily removed without damage by unscrewing the aluminum retainer extrusions. This permits easy

access into and through walls for maintenance, laboratory modification, or replacement. Some details can be seen in the photos which appear with this REPORT. The most critical area in the labora tory has proved to be around the sink, where water or dilute acid may be splashed onto the wall surface. Many laminates cannot withstand this abuse, and thus any candidate materi al should be subjected to some labora tory testing to judge the durability of the finish. The latest materials that have been used at NBS have displayed good chemical resistance, but only moderate resistance to staining. Where the floor layout permits, some wall panels may be attached directly over the original laboratory wall, again using the aluminum retaining extru sions. It might be advisable to paint these extrusions in some laboratories, but this additional expense does not seem to be warranted unless very high concentrations of acid fumes are en countered.

Entry Doors, Pass Throughs, and Windows Two of the early NBS clean rooms utilized change rooms and air showers for entry to and exit from the clean room. The present design utilizes nei ther facility. Although an antecham ber exists that may seem to serve this function, it is actually for the purpose of accommodating visitors to the clean lab. Glass windows in the partition walls permit visitors to observe the laboratory and provide an easy means of checking on the safety of the labo ratory occupants. With the present design, there is no reason why the entry door could not lead directly to the outside corridor of the building. Thus, although the antechamber is convenient for storage, it could be eliminated altogether to create more laboratory space. Measurements of the air quality in the old clean rooms and the new clean labs show that no degradation of lab . air quality occurs due to the elimina tion of the air shower entry to the clean lab. Entry to the laboratory is in street clothes. Also, the elimination of the pass through for equipment per mitted an additional economy and greater usable laboratory space. Rea sonable precautions, such as the pres ence of sticky mats at entrances, are taken to minimize difficulties upon entering the clean lab. The usual clean room garments are kept in the labora tory and put on after entry. For reasons of economy as well as freedom from trace element contami nation, all entry doors are of wood ve neer hollow-core construction. Con ventional hardware is used because of low initial cost and ease of replace

ment. More exotic and expensive ma terials are available, however. Entry doors are located directly in line with hallway doors for two principal rea sons. First, this arrangement provides an easy unrestricted exit from the lab oratory in the event of an emergency. As shown in the diagram of the clean room, Figure 3, these doors are 0.76m-wide (2.5 ft) double doors for a total span of 1.5 m (5 ft). Second, this pro vides access to the clean laboratory for the installation or removal of equip ment. Ordinarily a smaller entrance, 0.76 m (2.5 ft) wide, is used to limit the amount of air lost from the laboratory. A design feature of the laboratory is that an open door does not change the volume or flow of clean air within the laboratory nor does it cause a drop in air pressure in the laboratory. For rea sons of safety, all of the entry doors contain a glass view port to permit the observation of laboratory occupants from the hallway. There are situations in which more conventional clean room designs are useful and even necessary. Our inten tion has always been to provide the maximum usable laboratory space for trace element chemistry for the avail able money. The compromises and changes in design described here have all been thought out for the particular application of trace element chemistry and then proved out or improved upon by the difficult process of trial and error.

Sinks, Faucets, and Other Services Services for the laboratory may be brought in through the inside of the air return walls or through trenches under the floor. Our most recent labo ratories have PVC faucets for distilled water, cold water, hot water, and vacu um lines. Building code regulations have prevented the replacement of the conventional natural gas valves. A considerable amount of the plumbing going into the laboratory can be con verted to plastics. PVC can be used for tap water, while polyethylene or Tef lon should be used for distilled water. In general, metal piping should be avoided. Glass, PVC, or other plastics may be used for waste drains. As with the natural gas valve, no suitable sub stitute for the code-approved soft iron piping for natural gas has been found. Plastic materials other than PVC may be available for laboratory fau cets and valves. At least one company produces a high molecular weight polyethylene valve. In our laborato ries, the distilled water brought into the laboratories is not used directly, but is redistilled before use. Thus, the PVC fixtures for distilled water are less of a problem than they would be otherwise. A large variety of polypro

pylene or polyethylene sinks, drain board and sink combinations, and cup sinks are commercially available, and these should be substituted for the more conventional stainless steel or stone sink and drain boards. The number of services and sinks placed in the laboratory should be kept to a minimum, partly to reduce the potential for laboratory contami nation and partly to confine the use of these services to specific areas of the laboratory. The electrical service is the one area in which almost no im provements have been made. Plastic cover plates are available, but the more common outlet box and armored cables have shown poor resistance to corrosion. So-called corrosion-resis tant devices have not proved to be much better. The most acceptable, though not perfect, solution found to date is to use an aluminum box designed as a floor-mounted telephone junction box. A single duplex outlet can be mounted on each side and the finished installa tion is seamless, smooth, and reason ably resistant to corrosion. A few of these boxes have been coated with a vinyl plastic material designed for coating tool handles.

Testing and Acceptance of a Clean Laboratory Most of these tests are performed according to Federal Standard 209b (9) requirements. Two basic pieces of equipment required are a di-octyl phthalate (DOP) smoke generator and a multichannel particle analyzer, which counts particulates in discrete size bands, e.g., 0.3-0.5 jum, 0.5-1.0 μηι, etc. Tests should be both static (test equipment only) as well as dy namic (laboratory in full operation, complete with people). Although stat ic tests are useful measures of a clean room's ability to deliver filtered air to a given point, the results do not indi cate how well the laboratory will work. Thus, the most meaningful particu late test of a clean lab will be those particulate counts performed under actual working conditions. The actual points of air quality measurement should be at locations where an open sample would be located. The center bench and exhaust hoods are exam ples. A high quality of filtered air measured a few inches below the HEPA filter is not necessarily a good assurance of similar air quality at the point of use. Particulate measurements made downstream from people or particulating machinery are going to be high in any clean room. Thus, it should not be surprising to count a sudden burst of particles when people pass in front of the sampler probe for the particle analyzer. It is our contention that a


good laboratory design allows for the rapid exhaust of such particles and manages the airflow in such a manner that this air does not pass over a sample. The side benches in the NBS design allow for the installation of work sta tions (laminar-flow fume hoods and clean-air benches, shown in Figure 3) that are isolated from the exhausting "dirty" air. Actual counts of particu lates from 0.3 to 100 μπι in one NBS clean laboratory have never exceeded 0.35 particles/L (10 particles per ft3) even under dynamic operating condi tions, and even when measured at the dirtiest locations in the laboratory, such as the floor or on the side walls adjacent to the exhaust grilles. The single exception involves particles generated by people passing in front of the sampler probe. The smoke generator is used to pro vide a very severe challenge to the HEPA filter system. This is useful in determining whether there are leaks through the HEPA filter or its seals, or if there are other routes of air infiltra tion. The smoke makes leak detection simple, since even a small leak will produce easily detectable numbers of particles. Without a smoke generator a small leak produces such a small num ber of particles that a significant amount of time is required to obtain a reliable particulate concentration. The situation is analogous to counting lowactivity radioactive sources. Since these tests confirm the partic ulate management of the laboratory, they are truly good only for confirm ing compliance with Federal Standard 209b. As has been mentioned before, this is an inadequate test for a clean chemistry laboratory. Thus, the real test of the working ability of the labo ratory is measured by blank levels themselves. At NBS we have chosen Pb as the element for these tests, since it is associated with air particulates and because a high-accuracy isotope dilu tion-mass spectrometry measurement system is available at NBS. For our laboratory, we have arbitrarily set a limit of 1 ng Pb/cm 2 of exposed sur face in a 24-h period under dynamic use conditions. Several other elements could be specified for this test; however, one should be reasonably sure that the re quirement specified for this test can be met by an actual operating labora tory. The experience of a working lab oratory may be the best guide in speci fying a target blank value for the labo ratory. Pairs (one open and one closed) of clean 30-mL Teflon beakers are placed in the sampling location for 24 h, and the difference in their blanks is judged to be the laboratory blank. Once the laboratory is actually in op eration, the total chemical blank

should be continuously monitored. Blank contamination might come from a number of sources other than the laboratory. Once the blank has ex ceeded normal levels, steps can be taken to ascertain the source of con tamination. The present laboratory has never been implicated as the source of contamination when high blanks have been encountered.

Application of the Analytical Clean Laboratory In evaluating how to use a chemis try clean lab, its limitations should be known in advance. Controlling the en vironmental blank caused by air par ticulates will be of little value if some other source of contamination, such as reagent contamination, is out of con trol. Clean rooms and laboratories will not remove gaseous contamination. Volatile substances can be expected to go right through the HEPA filter in the gaseous state, tetraethyl Pb being one example. Fortunately, there are relatively few instances where this is a problem. Usually, sample acquisition has been performed by someone other than the analyst, whose first job may be subsampling or obtaining a representative cross section of the entire sample. Quartering operations are easily per formed on a clean bench. The sam pling operation might be as simple as weighing out a portion of the sample on an analytical balance, which can be easily performed in the clean lab. Next, various devices for sample drying including furnaces, convention al ovens, vacuum ovens, or freeze dryers might be used. Sample dissolu tion techniques vary, but wet acid di gestions are the type most frequently employed at NBS. The vertical lami nar-flow clean-air fume hood has be come essential for this operation. Other types of bomb or sealed com bustion approaches have been used and here, too, the clean lab is useful for the loading and unloading opera tions. The need for further sample prepa ration will depend upon the instru ment requirements. For mass spec trometry, the elements of interest must be separated from the matrix. These chemical separation techniques might include ion exchange chroma tography, solvent extraction, precipi tation and filtration, electrodeposition, or even distillation. With some thought, most chemical operations that take place between sample acquisition and presentation to the instrument can be adapted to the clean lab. Admittedly, some pieces of apparatus are particularly dirty, ovens and furnaces being good exam ples. Thus, apparatuses of this type


should be carefully selected to mini mize contamination. In addition to sample manipulation, many peripheral operations that sup port the analytical effort are carried out in the clean laboratory or nearby clean facilities. The final stages of beaker or apparatus cleaning are best carried out under Class 100 condi tions. The subsequent drying of this apparatus may be done by leaving the apparatus under the moving air stream of the clean air bench. Even the ultrapure distilled water is best prepared in the same laboratory where it is being used. The preparation of many purified reagents requires Class 100 conditions, but many of these should be in a separate facility. Final ly, the clean lab has been used for the actual preparation and packaging of a number of Standard Reference Mate rials (SRMs). The primary use of the clean labs has been to provide analytical support for the SRM program. Virtually every analytical group at NBS has made use of the clean rooms and laboratories since 1971. More than 200 SRMs and nearly all trace element standards re leased by NBS since approximately 1972 have been at least partially ana lyzed utilizing the clean laboratory. This has included all types of samples such as steels, alloys, ores, rock, water, particulates, botanicals, biological and clinical samples, environmental sam ples, and waste stream and effluent materials. It is not an exaggeration to say that many SRMs either could not have been analyzed or would have had reduced value as standards if the clean laboratories and purified reagents (14, 15) had not been available. Direct reference to specific labora tory blank levels has been avoided since such information is both sparse and potentially misleading. Not all el ements exhibit the same response to these control techniques. However, to cite Pb as a typical heavy metal, in 1970 the total chemical processing blank for mass spectrometry due to reagents, apparatus, and laboratory contribution was about 300 ng. By the mid-seventies this blank had been re duced to about 2-5 ng. Absolute blank levels are difficult to compare since the size of the sample, type of process ing, and duration all affect the size of the blank. The most important figure to note is the relative improvement in the blank. The laboratory blank reduction may vary in magnitude from element to el ement and will certainly vary with the particular type of chemistry, but the net result is nearly always a dramatic improvement in the system analytical blank. The need to analyze the lunar samples was cited as the original rea son for building the clean lab at NBS.

The lunar samples were analyzed for Pb, U, Th, K, Rb, and Sr, and these analyses could not have been per formed with any reliability under or dinary laboratory conditions (12).

Other Applications of Clean Air Design One of the laboratories at NBS, the NBS National Environmental Speci men Bank (16), is split into a dualpurpose trace element and trace or ganic lab. The two laboratories coexist side by side, each optimized for its specific job. For the trace organic lab this meant the use of glass instead of plastic view screens, stainless steel in stead of laminate counter tops, and charcoal air filtration as well as HEPA air filtration. This latter combination is the first such application for trace organic chemistry at the NBS. The en tire laboratory was designed to oper ate in conjunction with the adjacent clean air room (not Class 100), which contains both mechanical and liquid nitrogen freezers. The laboratory and storage facility had to be built to as sure the lowest possible contamination during the analysis and long-term storage of specified sample materials. The newest laboratory, the Toxic Chemicals Handling Facility, is de signed for the safe handling of toxic samples and materials including car cinogens. This laboratory utilizes both charcoal and HEPA air filtration. Like a clean room, the air is forced to travel in a well-defined path, but unlike a clean room the primary emphasis here is on protecting the chemist. The pri mary containment for the samples is in a conventional laminar-flow biohazard hood. Further details on this lab oratory should be available in the near future from the NBS Organic Analyti cal Research Division. Another recently completed clean laboratory is the NBS Pure Reagents Laboratory (14). This facility pro duces about 250-L/y of ultrapure acid and solvents by nonebullient distilla tion originally described by Kuehner et al. (15). The greatest problem for this laboratory was the maintenance of Class 100 air quality under the ex tremely adverse condition of corrosive fumes. Again, the solution is proper control of clean air and exhaust of clean air in the laboratory. Details of the laboratory operation are different from clean room operation, but plenti ful Class 100 air is an essential ele ment in its success. A number of other laboratories in the Inorganic Analytical Research Di vision also utilize clean air to differing degrees. Several laboratories have simple clean benches for limited oper ation in a confined area. One laborato ry under construction and another being planned could be classified as

semiclean laboratories, probably Class 1000 or Class 10 000. Both of these de signs are of relatively low cost and pro vide a graduated approach to the pro vision of clean air facilities. Not every operation will warrant a Class 100 clean lab, and the provision of a full spectrum of facilities provides for the maximum work throughput at the lowest cost. Having a graduated range of clean lab facilities (e.g., Class 100 to Class 10 000) also assists in properly segregating inappropriate tasks from those requiring the most stringent conditions. These comments have focused on those aspects of clean lab design that are most likely to be of interest to the chemist. Other engineering aspects are equally amenable to solution, but are beyond the scope of this discussion. During the remainder of the eighties we might expect to see clean laborato ries become much more common and less of a curiosity. Many of the institu tions that have built one clean labora tory have suddenly discovered the need for additional clean laboratory space. It is hoped that this article will stimulate thought among those con templating a similar laboratory.

Room and Work Station Requirements, Controlled Environments"; Government Services Administration Business Ser vice Center, Boston, Mass., 1966. (9) Federal Standard No. 209b; Govern ment Services Administration Business Service Center, Boston, Mass., 1973. (10) Federal Standard No. 209; Govern ment Services Administration Business Service Center, Washington, D.C., 1963. (11) Mitchell, J. W. Anal. Chem. 1973,45, 492-500A. (12) Barnes, I. L.; Carpenter, B. S.; Gar ner, E. L.; Gramlich, J. W.; Kuehner, E. C; Machlan, L. Α.; Maienthal, E. J.; Moody, J. R.; Moore, L. J.; Murphy, T. J.; Paulsen, P. M.; Sappenfield, Κ. Μ.; Shields, W. R. "Isotopic Abundance Ra tios and Concentrations of Selected Ele ments of Apollo 14 Samples", Proceed ings Apollo 14 Lunar Science Confer ence, Geochim. Cosmochim. Acta, Suppl. 3,1972,2,1465-72. (13) Laboratory of the Government Chem ist (London), "Laboratory Fume Extrac tion Ductwork—Method of Construc tion"; LGC No. 1614, private communi cation, 1982. (14) Moody, J. R.; Beary, E. S. Talanta 1982, in press. (15) Kuehner, E. G; Alvarez, R.; Paulsen, P. J.; Murphy, T. J. Anal. Chem. 1972, 44, 2050-56. (16) Harrison, S. H.; Zeisler, R; Wise, S. Α., Eds.; "Pilot Program for the National Environment Specimen Bank—Phase I"; Document No. PB81-173-320; National Technical Information Service, Spring field, Va.. 1981.

Acknowledgment The author wishes to recognize the efforts of William R. Shields and War ren M. Dexter in designing and build ing the first NBS clean laboratory in 1971. In addition, I. Lynus Barnes of NBS was instrumental in the con struction of the improved clean labo ratory in 1975-76.

Certain commercial equipment, instruments, and materials are identified in this report to spe cify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Stan dards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

References (1) Murphy, T. J. "The Role of the Analyt ical Blank in Accurate Trace Analysis"; Proceedings of the Seventh Materials Research Symposium; U.S. Government Printing Office, Washington, D.C., 1976; pp. 509-39. (2) Patterson, C. C; Settle, D. M. "The Reduction of Orders of Magnitude Er rors in Lead Analysis of Biological Mate rials and Natural Waters by Controlling the Extent and Sources of Industrial Lead Contamination Introduced during Sample Collecting, Handling, and Analy sis"; Proceedings of the Seventh Materi als Research Symposium; U.S. Govern ment Printing Office, Washington, D.C., 1976; pp. 321-51. (3) Zief, M.; Mitchell, J. L. "Contamina tion Control in Analytical Chemistry"; Wiley: New York, 1976. (4) Zief, M.; Nesher, A. G. Environ. Sci. Technol. 1974,8,677-78. (5) Gilbert, H.; Palmer, J. H. "High Effi ciency Particulate Air Filter Units"; TID-7023, USAEC, Washington, D.C., 1961. (6) Langmuir, I. "Filtration of Aerosols and Development of Filter Materials"; O.S.R.D. Report No. 865; Office of Tech nical Services, Washington, D.C., 1943. (7) Whitefield.W.J.'TheDesignofa Dust-Controlled Vented Hood Utilizing Laminar Air Flow"; Research Report No. SC-4905(RR); Sandia Corp., Albuquer que, N.M., 1963. (8) Federal Standard No. 209a, "Clean


John R. Moody is a research chem ist in the Center for Analytical Chem istry of the National Bureau of Stan dards. He received his undergraduate education in chemistry at the Univer sity of Richmond and earned a doc toral degree in analytical chemistry at the University of Maryland. In ad dition to clean labs and ultrapure re agents, Moody's research interests in clude trace and ultratrace analysis and high-precision titrations.

NBS Clean Laboratories for Trace Element Analysis - Analytical

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