1. The Field of the Invention
The present invention relates to an apparatus and method for the measurement of submicrogram amounts of mercury by flameless atomic absorption spectrophotometry; more particularly, the invention relates to a one piece mercury absorption-reduction cell which can be fitted within the absorption chamber of an atomic absorption spectrophotometer.
2. The Prior Art
Science has long recognized the toxicity of mercury and that mercury can be taken into the body through the food we eat, the water we drink or even the air we breathe. Although man and animals are rarely exposed to mercury in concentrations great enough to cause immediate physical effects, mercury accumulates within the body until it reaches toxic levels.
Unfortunately, modern technology and industrial operations have increased the potential for exposure to mercury because many manufacturing processes emit mercury vapors into the working environment. For decades, the literature has contained reviews of the toxic effects of industrial exposure to mercury. See M. C. Battigelli, "Mercury Toxicity from Industrial Exposure: A Critical Review of the Literature--Part II, " 2 J. Occup. Med. 337 (1960); R. G. Smith, A. J. Vorwald, L. S. Patil, and T. F. Mooney, "Effects of Exposure of Mercury in the Manufacture of Chlorine," 31 Am. Ind. Hyg. Assoc. J. 687 (1970). In light of the dangers of mercury poisoning, much attention has been directed to the development of methods for measuring mercury concentrations in the human body.
Today, there is general agreement that blood mercury levels can be used as an estimate of the recent exposure and that the amount of mercury in the urine can be an indicator of mercury accumulation in the kidney. See G. M. Cherian, J. B. Hursh, T. W. Clarkson, and J. Allen, "Radioactive Mercury Distribution in Biological Fluids and Excretion in Human Subjects after Inhalation of Mercury Vapor," 33 Arch. Environ. Health 109 (1978). It has been found that by monitoring mercury levels in urine at the end of each work week, it is possible to ascertain mercury exposure of employees exposed to mercury vapors during the previous week. See W. Stopford, S. D. Bundy, L. J. Goldwater, and J. A. Bittikofer, "Microenvironmental Exposure to Mercury Vapor," 39 Am. Ind. Hyg. Assoc. J. 378 (1978). Hence, many industries where there is potential for mercury exposure have initiated screening techniques as a means of protecting their employees. However, in order to obtain meaningful information at a reasonable expense, these industries require an accurate, yet quick, method for monitoring mercury levels in blood and urine samples.
While several methods for determining trace amounts of mercury have been developed, the generally accepted and most widely used method was developed in 1970 by Uthe et al. and comprises a complicated apparatus which is used in conjunction with an atomic absorption spectrophotometer. See J. F. Uthe, F. A. J. Armstrong and M. P. Stainton, "Mercury Determination in Fish Samples by Wet Digestion and Flameless Atomic Absorption Spectrophotometry, 27 J. Fish. Res. Bd. Canada 805 (1970). The basis of this NIOSH-approved procedure rests in the ability of mercury to be reduced in solution to its elemental state by the addition of a reductant, typically a stannous salt such as stannous sulfate. The mercury in the vapor phase is then swept from the air space above the solution into a cell located in the absorption compartment of an atomic absorption spectrophotometer. The absorbance of mercury at 253.7 nm is then measured as a function of the concentration of mercury in the sample.
The relationship between the mercury concentration and the light absorption is generally established by one of two methods. For the first method, a set of samples of different known concentrations of mercury are run through the system. The absorbance for each sample is noted, and a callibration curve can then be easily prepared in order to correlate absorbancy to mercury concentration. The second method utilizes the process of standard additions to determine the amount of mercury in a sample.
A common embodiment of the Uthe apparatus (which is generally depicted in FIG. 1) basically consists of a reduction chamber which is usually in the form of a flask into which a sample of 25 ml or more is introduced. A reductant is then added to the sample and the mixture is thoroughly mixed with a magnetic stirrer for several minutes. Air is then forced through the reduction chamber to sweep the mercury laden gases into a second chamber located in the absorption compartment of an atomic absorption spectrophotometer.
A modified version of the Uthe apparatus replaces the magnetic stirrer with a bubbler inserted into the sample. The bubbler provides the dual service of mixing the sample and reductant and also providing the air to force the mercury laden vapors into the absorption chamber.
The absorption chamber of the Uthe apparatus is generally a cuvette with quartz lenses located on the ends. The concentration of the mercury in the vapor is then measured, and the gases are vented to a scrubber. Usually, a dessicant tube is located between the reduction and absorption chambers to remove water vapors so that they do not fog the absorption compartment, thereby creating erroneous results. Alternatively, the absorption chamber may be sufficiently heated to prevent fog formation.
The Uthe prior art apparatus has greatly improved the ability to obtain the sensitivity needed to detect trace amounts of mercury. However, there are several disadvantages and problems associated with the apparatus. First, a relatively large sample (i.e., 25 ml or more) is typically needed to perform the measurement. Normally, 25 ml is the total amount of sample which is available from a single digest. Thus, it is difficult, if not impossible, to perform repetitive tests to verify the data obtained by the Uthe method. Moreover, in many applications, the Uthe method is very impractical because the specimen sizes are often so small (such as with atmospheric fallout and tissues from small animals) that a suitable sample cannot be prepared.
Second, the Uthe apparatus requires positive pressure to sweep the mercury laden vapors from the mixing flask to the cuvette. Accordingly, if there are any leaks within the system, this positive pressure will force the toxic mercury vapors into the atmosphere, thereby subjecting the laboratory technician performing the measurements to the toxic mercury vapors.
Third, mercury is often absorbed onto the walls of the complicated apparatus and the extensive tubing connecting the flask and cuvette, thereby resulting in erroneous readings. Moreover, the mercury build up on the tubing creates problems in properly cleaning the apparatus, as well as reducing the number of tests which can be conducted in a given period of time.
In addition to these significant problems, the prior art Uthe apparatus is relatively expensive and occupies a significant amount of space in front of or on the side of the atomic absorption spectrophotometer. These problems have thus caused those skilled in the art to seek after better systems for analyzing trace amounts of mercury.
A method somewhat simpler than that of Uthe was described in S. Tong, "Stationery Cold Vapor Atomic Absorption Method For Mercury Determination," 50 Anal. Chem. 412 (1978). The Tong method utilizes a 4 cm ultraviolet ("UV") cell. According to this method, both the sample and the reductant are mixed together within the UV cell. The cell is then placed within the atomic absorption spectrophotometer and the mercury concentration in the air space of the cell is measured.
This procedure also has several drawbacks. First, the UV cell must be removed from the spectrophotometer between samples to be cleaned and reloaded. The cell must be aligned within the light path of the spectrophotometer for every sample. Also, small droplets of the solution typically adhere to the quartz lenses on the ends of the cuvette, thereby interfering with the absorption reading.
In view of the foregoing, it would be a significant advancement in the art to provide an apparatus and method for measuring trace amounts of mercury in which measurements can be made on small samples so that replicate data can be taken with respect to a single digest. It would also be advantageous to provide an apparatus which is simple to use and easy to clean. A further advancement in the art would be to provide an apparatus which does not require the use of positive pressures with the attendant possibility of leaks which could contaminate the area in which the technician operates. Such an invention is disclosed and claimed herein.