1288
Anal. Chem. 7984, 56, 1288-1293
saturated NaNO, was added to oxidize any free I- to I,; the resulting 1, was extracted into the toluene. Three microliters of the toluene was injected onto the GC to measure the formed 1,; one milliliter of the toluene was counted with a hyperpure germanium low-energy photon spectrometer to determine the chemical yield. Typical gas chromatograms are shown in Figure 8. The results of these analyses are shown in Table 11. The precision of triplicate determinations was 12% a t one standard deviation. Although the determinations were within the 95% confidence interval, the analyses were 23 % higher than the nominal makeup value. The cause of this bias is unknown. On the basis of these results, it appears the technique has an accuracy of 23% and a precision of 12% at the one standard deviation level. Registry No. I,, 7553-56-2;AgI, 7783-96-2; CH31, 74-88-4;
CH3CH21,75-03-6; 2-propyl iodide, 75-30-9; 1-propyl iodide, 107-08-4.
LITERATURE CITED (1) Castello, G.; D'Amato, G.; Biagini, E. J. Chromatogr. 1989, 4 1 , 313-324. (2) Corkill, J. A.; Giese, R. W. Anal. Chem. 1981, 53, 1667-1672. (3) Fernandez, S.J.; Rankln, R. A.; McManus, G. J.; Nielsen, R. A,; Deimore, J. E.; Hohorst, F. A.; Murphy, L. P. "Determination of Low Specific Activity Iodine-129 Off-gas Concentrations by GC Separation and Negative Ionization Mass Spectrometry"; ENICO-1134, 1983, 11-29. (4) Patte, F.; Echeto, M.; Laffort, P. Anal. Chem. 1982, 5 4 , 2239-2247. (5) Miller, D. A.; Grimsrud, E. P. J. Chromafogr. 1980, 190, 133-135. (6) Keller, J. H.; Duce, F. A.. Maeck, W. J. "A Selective Adsorbent Sampling System for Differentiating Airborne Iodine Species"; CONF700816, Eleventh AEC Air Cleaning Conference, 1970; Vol. 2, pp 62 1-625.
RECEIVED for review June 15,1983. Accepted March 19,1984. Resubmitted February 21, 1984.
Collection and Determination of Volatile Organic Mercury Compounds in the Atmosphere by Gas Chromatography with Microwave Plasma Detection David S. Ballantine, Jr.,*and William H. Zoller' Department of Chemistry, University of Maryland, College Park, Maryland 20742
A method for the collection of two volatile organlc mercury compounds In the atmosphere Is described, uslng Chromosorb 101 as a collection substrate. The analytlcal method Involves direct eiutlon of the organic mercury compounds from the collectlon substrate onto a gas chromatographic column prlor to detection wlth a mlcrowave plasma detector. Methylmercury chloride (MMC) Is collected at ambient temperatures, and dlmethylmercury (DMM) Is collected by use of a cryogenic trap at -80 O C . Coilectlon efflclencles for MMC and DMM are 95 f 3 % and 96 f 2 %, respectlvely. The absolute detection llmlt of the system Is 0.05 ng, wlth a detectlon limit for real atmospheric samples of 0.1 ng/m'. Posltlve Identification of collected compounds Is achieved by comparison of sample elution volumes with standards.
A growing public interest in environmental quality has led
to the development of analytical techniques for the monitoring of environmental pollutants. Due to its acute toxicity and its tendency to bioaccumulate, mercury is of prime interest. Being extremely volatile in the organic and elemental forms, mercury is well dispersed in the atmosphere. The activity of certain bacteria, molds, and enzymes in the soil or sediment can produce methylated mercury from elemental or inorganic mercury (1-4). The organic mercury compounds produced, primarily dimethylmercury and methylmercury halides, are potentially more toxic than inorganic mercury forms. Therefore, recent studies of environmental mercury have been concerned with its chemical speciation to determine not only the amounts of mercury present but the chemical forms as well. More extensive data in this area will assist in deterCurrent address: INC-7, MSJ514,Los Alamos National Labo-
ratory Los Alamos, NM 87544.
mining the role of organic mercury in the global cycling of the element. Previous studies have been performed by using a method of selective preconcentration followed by pyrolysis and cold vapor atomic fluorescence detection to determine different mercury species collected from the atmosphere (5). Other studies have been performed by using sequential specific absorption tubes which separate different chemical forms of mercury by selective collection (6-8). In these latter studies, mercury compounds were thermally desorbed and re-collected on gold surfaces prior to elution into an emission detector. While these methods represent a significant advance in atmospheric mercury sampling by achieving the separation of volatile species of mercury, the analytical methods prevent positive identification of the compounds by converting all forms to elemental mercury prior to detection. Chromatographic substrates have been used successfully for the collection of organics and of organic mercury (5-10). By logical extension, a chromatographic method of analysis would permit the positive identification of organic mercury compounds by comparison of sample elution times/volumes with standard compounds. For this study, a collection method has been developed that is compatible with a chromatographic method of analysis and is capable of detecting levels of organic mercury in the atmosphere as low as 0.1 ng/m3. Since they are known to be produced by biogenic activity (1-4), dimethylmercury and methylmercury chloride were selected as standard compounds during the laboratory studies. These compounds were also used as model compounds in the development of the selective absorption tube system of Braman and Johnson (6),but because of their choice of analytical method, any organic compounds detected could not be positively identified and could only be operationally defined to be methylmercury chloride or dimethylmercury.
0 1984 American Chemical Society 0003-2700/84/0356-1288$01,50/0
ANALYTICAL CHEMISTRY, VOL. 56,
Table I. Testing of Substrates for the Collection of MMC substrate breakthrough volume collection efficiency 5% FFAP on Gas Chrom Q 20% Carbowax 20M on Supelcoport 3% OV-1 on Gas Chrom Q 3% Hi-eff 8BP on Gas Chrom Q Chromosorb 101 Chromosorb NAW Porapak P XAD-2 Tenax GC
CHART
low (3-4 L/cm) minimum retention no retention minimum retention high (2.5 m3/cm) minimum retention low medium low (3-4 L/cm)
CHROMW GRAPH
u u Figure 1. GC/MPD system used for the analysis of organic and total volatile mercury: (1) microwave cavity, (2) quartz focusing lens, (3) GC injection port, (4) power reflectancemeter, (5) photomukiplier power supply, (6) stable power source, (7) variable voltage source.
Samples and standards were analyzed by using the gas chromatograph/microwave plasma detector (GC/MPD) system diagramed in Figure 1. T h e GC had been modified so that samples or standards collected on suitable substrates could be thermally desorbed directly onto the GC column. The plasma emission detector has the advantage of being extremely selective for mercury when set for the 253.7-nm Hg emission line (11).
EXPERIMENTAL SECTION Development of the Collection System. The primary considerations in the selection of a suitable collection substrate were (1)good collection efficiency, (2) large breakthrough volume, (3) good elution characteristics, and (4) economic availability. For this study, collection efficiency was determined by comparison of peak areas of standards collected on selected substrates and subsequently eluted into the GC vs. peak areas of standards injected directly onto the front of the GC column. The primary intention of this study was to develop a collection method that was relatively simple and compatible with the analytical system. The size of the glass tubing used in preparing the collection tubes was selected because it could be conveniently inserted into the analytical gas stream, and samples could be eluted directly onto the GC column. By use of a method of direct elution onto the column, the need for involved extraction of samples from collection tubes is eliminated, as is the possibility of introducing contamination from solvents and sample handling. Breakthrough volume is defined as the volume of sampled air that would carry the compound of interest completely through a tube containing a given amount of substrate. In the field, the breakthrough volume represents the volume of air that can be sampled before the compound(s) of interest are lost through the tail end of the collection medium. This volume is highly dependent on the environmental and sampling parameters such as ambient collection temperature and flow rate. Glass tubing dimensions were 6.4 mm 0.d. and 4 mm i.d. Since the flow rate through the tubing is a function of the thickness of the packing bed, the resulting breakthrough volumes are operationallydefined in terms of m3 (air)/cm of packing in the collection tube. A material possessing good elution characteristics will release the collected compounds cleanly and rapidly upon heating, without
NO. 8, JULY 1984
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elution characteristics
good (> 95%)
excellent; clean sharp peaks
good (> 95%)
poor; broad peaks, tailing
fair good (>95%)
poor; broad peaks excellent; clean sharp peaks
losses due to decomposition or irreversible adsorption. Each of these considerations will be discussed further with regard to the collection of methylmercury chloride (MMC) and dimethylmercury (DMM). MMC. Since organic mercury compounds are very volatile, each of the substrates listed in Table I was first tested for breakthrough volumes, Breakthrough volumes at room temperature were determined by placing a piece of 6.4 mm glass tubing containing a given amount of substrate into the gas stream of the GC/MPD system. A given amount of standard solution was then injected onto the front end of this tube. From the elution time and the flow rate of the argon carrier gas the breakthrough volume was calculated. The tube was then wrapped with heating tape and the process repeated a t various temperatures to determine the relationship between breakthrough volume and temperature. At the same time the elution characteristicsof each substrate were determined by observing the shape of the resulting standard peaks. Several of the substrates tested failed to retain MMC at 25 "C. Others exhibited retention volumes of approximately 3-4 L (air)/cm packing which is too low for use as a collection substrate in the field. Of the substrates tested, only Chromosorb 101 exhibited a breakthrough volume large enough for collection of expected background levels of MMC. The results for breakthrough volume, collection efficiency, and elution characteristics are summarized in Table I. By plotting breakthrough volume vs. temperature, we determined the breakthrough volume of newly conditioned Chromosorb 101 a t 25 OC to be 2.5 1m3/cm (packing) (see Figure 2). The large error is due to extrapolation to 25 "C, using breakthrough volumes determined at elevated temperatures. When this error is taken into consideration, collection tubes containing 4-6 cm of Chromosorb 101 would possess a breakthrough volume of between 6 m3and 19 m3. Because of the long time periods required to pass such large volumes of air through the tubes, no attempt was made to observe breakthrough at 25 "C. Actual sample volumes in the field were all below 6 m3. Chromosorb 101 exhibited poor elutiop characteristics, with resulting peaks being broad with significant tailing. This broadening increased with each subsequent use of a given tube, and longer periods of time were required to Completely desorb the MMC. The elution characteristics could be improved by reconditioning the tubes with a silylating agent, but only temporarily. This problem was circumvented by re-collecting MMC desorbed from Chromosorb 101onto 5% FFAP or Tenax GC prior to elution onto the GC column. Both 5% FFAP and Tenax GC exhibited excellent elution characteristics with resulting peaks being relatively sharp with minor tailing. Economic factors favor the use of 5% FFAP, which is significantly cheaper than Tenax GC, although Tenax GC has a higher thermal stability. The collection efficiency of Chromosorb 101 was then determined, as described previously. Under laboratory conditions (25 O C , relative humidity 20-4070) the collection efficiency was determined to be 95 & 3%. The collection efficiency was then tested under conditions of higher H 2 0 vapor. A standard solution was injected onto the front end of a collection tube and the tube attached to an oilless diaphragm pump. A volume of air less than the breakthrough volume was bubbled through a vessel containing water before being pulled through the collection tubes. While the actual relative humidity was not measured, these steps were taken to determine breakthrough volumes and collection efficiencies under simulated conditions of water-saturated air. Subsequent analyses determined the collection efficiency of
*
1290
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
I
I
I
I l l 1 1 1 1 1
I
I
I
I I I I I
other mercury compounds were tested for retention or lack of retention by the drying tube. Originally, indicator Drierite was used as a drying agent to monitor the degree of saturation during breakthrough volume and collection efficiency determinations. Subsequent analysis of collection tubes during this phase of the study resulted in the introduction of a volatile substance into the detector which fouled the capillary tube and extinguished the plasma. This compound, believed to be a component of the indicator, eluted prior to and interfered with the detection of DMM. Subsequent use of nonindicating Drierite alleviated this problem. Dimethyl mercury passed through the Drierite without measurable retention regardless of the degree of saturation of the drying tube. Total Volatile Mercury. Total volatile mercury samples were collected on gold-coated glass beads. The glass beads were prepared by dissolving Au(m) or AuC13 in aqua regia. Glass beads that had been etched with dilute HF were added to the solution, and the volume of the mixture was reduced by heating. The resulting glass bead/gold chloride slurry was packed into a glass tube and heated to 350-400 "C while flushing the tube with H,(g). Total volatile mercury samples were collected by drawing air through blanked gold-coated glass bead tubes. Blanked quartz fiber filters were placed in front of the collection tubes to remove particulate matter from the sampling stream. These filters were not analyzed for particulate mercury. The integrity over time of organic mercury samples was determined by injecting standards onto tubes and storing them at room temperature and under refrigeration (5-10 "C) for a period of 6 months. Of primary concern was the possibility of conversion from organic mercury to Hgoduring storage. Subsequent analyses of tubes containing DMM that had been stored at room temperature resulted in the appearance of a large spike of mercury eluting into the detector unretained by the Chromosorb 101 column. Since other compounds studied (MMC, DMM) were retained by Chromosorb 101 while Hgo was not, this was taken as an indication that conversion of DMM to Hgo had occurred on these tubes. Analysis of DMM tubes stored under refrigeration showed no evidence of such conversion. Tubes containing MMC revealed no losses due to conversion when stored at room temperature or under refrigeration. Analytical System. Analyses were performed on a modified gas chromatograph equipped with a microwave argon plasma detector (Figure 1). The detector system is similar to the setup used by Talmi (12). Injection ports of the GC and Swagelok fittings were all cored to allow for the passage of 6.4-mm glass tubing and reduce the possibility of mercury losses on exposed metal surfaces. Eluents from the chromatographic column exhausted into a quartz capillary tube (i.d. 1.0 mm, 0.d. 6 mm) leading into the detector. The detector consists of a quartz capillary tube passing through a three-fourths wave cylindrical microwave resonance cavity (Opthos Optical) coupled to a microwave power generator (Raytheon). A Tesla coil provided the spark to ignite the plasma, and the image of the plasma was focused on the entrance slit of a Heathkit monochromator tuned to the 253.7-nm Hg emission line. During analysis, the T1 position was occupied by a collection tube of either Chromosorb 101 (MMC, DMM) or gold-coated glass beads (total volatile mercury). The T2 position was occupied by a preconcentration tube consisting of a clean, blanked tube packed with the same material (e.g., Chromosorb 101 or gold-coated glass beads). Both T1 and T2 were wrapped with heating tape and could be alternately connected to a variable voltage supply to thermally desorb the collected samples. Optimum parameters for the analysis of MMC and DMM were determined by observing the change in resolution and detector response as a function of column temperature and flow rate. Since analyses for MMC and DMM were performed on different columns, the optimum flow rate differs for these two systems. A higher flow rate was used for the analysis of MMC to reduce the amount of tailing on eluting peaks. Since peak tailing was not a problem during DMM analyses, the optimum flow rate was slower. The use of an empty glass column when analyzing total volatile mercury tubes permitted the use of lower flow rates, which increased residence time in the plasma and increased detector response. At very low flow rates, however, the plasma became unstable and results were irreproducible. An optimum flow rate
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8,JULY 1984
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9-
Table 11. Analytical System Parameters for the Analysis of MMC, DMM, and Total Volatile Mercury e--
System parameter flow rate, mL/min oven temp, "C column (1 (1 m x 6 mm 0.d.) collection tube ( T l )
MMC 85+5 165
i-
5
DMM
total volatile Hg
75r 5
60
same
same
i-
7--
5 6--
H5 nl,"
5% FFAP on Gas Chromosorb 101 (60/ Chrom Q (SO/ 80 mesh) 1 0 0 mesh)
empty glass column
Chromosorb 101 Chromosorb gold-coated glass (4-6 cm) (60/80) (2-4 beads cm) preconcn 5% FFAP on Gas Chromosorb gold-coated 101 ( 5 cm) glass tube (T2) Chrom Q (9 beads cm) cm) Tenax GC (60/ 80 mesh) ( 5 cm) 350 desorption 200 90 temp, "C Detector
microwave power 26 W (forward), slit width 110 pm
