the dependent variable of the linear regression equation; the total number of counts observed in a time t. X the independent variable of the linear regression equation; the observation number which is directly related to time. c the intercept of the linear regression equation on the Y axis. Here c x for stable, drift-, and oscillation-free
y
=
=
variance (. This has been proved for vs but is not necessarily true always for , although it is used here as such. (2 = variance of e.
,2
=
=
data. m = the slope of the linear regression equation. Here has a value close to m = 0 or m < c for drift-free data. = sample size, number of observations. 2 = an MSSD method statistic.
ts
=
a
=
(n
—
2)/[(«
+
1)
(n
—
1)]
Student t statistic. ts
m
e/ ,
=
or
ts
=
[1
-
( .,2/2)]/ ,
Significantly large positive values of ts indicate drift and significantly large negative values indicate oscillations. tx = [1 {vx/2)]dt. In conjunction with ts, tx can confirm drift or oscillations, or indicate drift plus oscillations. -
-
For
(x, +
=
'2
a
—
[
x,)2/(
—
random sample from
1), a
i
=
1,2
...
(n
—
1)
normal distribution, the
average value of
Downloaded via UNIV OF NEW ENGLAND on January 17, 2019 at 19:57:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
2
=
2 2.
52/s2 and for a random sample from a normal distri2. Values of vs2 significantly lower than 2 bution vs indicate drift and values significantly greater than 2 indicate oscillations. Vx 2/ and for a random sample from a normal distribution vx 2. e 1 (tj/2) and for a random sample of n observations > 25 from a normal distribution, c is nearly where normally distributed with an average value of 0 and =
Vs
=
=
=
=
-
ACKNOWLEDGMENT We wish to thank Harry B. Whitehurst for helpful dis-
cussions and Sharon Ode for some of the data. Thanks also are due to Dave Delaney of Hewlett-Packard, 2336 East Magnolia Street, Phoenix, Ariz. 85034 for programming help and from whom copies of our HP-9810A statistics program may be obtained.
Received for review February 22, 1973. Accepted May 23, 1973. From a thesis submitted by one of the authors (K. H.) to the Graduate Faculty of Arizona State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry.
Atomic Absorption Determination of Elemental Mercury Collected from Ambient Air on Silver Wool S. J. Long, D. R. Scott, and R. J.
Thompson
Quality Assurance and Environmental Monitoring Laboratory, Environmental Protection Agency, Research Triangle Park, N.C. 27711
A flameless atomic absorption system is described to analyze ambient mercury levels from 15 ng/m3 to 10 use of in-series silver wool collectors for 24hour sampling times. Absorption-time area measurements gave better precision at higher mercury concentrations and calibration curves extending over wider ranges than did absorption-peak height measurements. The mercury vapor calibration curves extend from 0.5 ng to 594 ng mercury and were reproducible to within 11% at 0.5 ng and to within 3%, relative standard deviation, beyond 6 ng. Detection limit was 0.3 ng. The effects of analytical train flow rate, the capacity of the silver wool collectors, and the release temperature for collected mercury were investigated. Ambient levels of dimethylmercury, sulfur dioxide, hydrogen sulfide, and nitrogen dioxide did not seriously interfere with the collection or analysis procedures.
pg/m3 by
In order to monitor ambient levels of elemental mercury, it is necessary to have a mercury collection system which is simple, rugged, and selective. The analysis system, located in a central laboratory, must be sensitive, reproducible, accurate, rapid, and free from ambient air in-
terferences. In addition, the analysis system must be capable of handling potentially wide ranges of mercury levels, from about 15 ng/m3 for urban locations (1, 2) to 10 µg/m3 levels near chlor-alkali plants. For a 24-hour sampling period at a collection flow rate of 100 cm3/min, this aerometric range corresponds to 2.1 ng to 1.4 µg of collected mercury. Our previous experience with liquid collecting media—e.g., acidic potassium permanganate and particularly iodine monochloride (3)—showed serious problems at mercury levels below 100 ng—i.e., below 300 ng/ m3. These problems include variable blank levels, poor precision of the flameless atomic absorption methods when applied to these solutions, instability of mercurystandard solutions at low concentrations, and instability of the collecting media after use. For these reasons, a scheme was required to collect elemental mercury over a wide range of ambient levels on a solid collector and to analyze the collected mercury via a very sensitive, selective, and reproducible method capable of operation over a wide range of mercury levels. (1) S. H. Williston, J. Geophys. Res.. 73, 7051 (1968). (2) R. S. Foote, Science. 177, 513 (1972). (3) A. L. Linch, R. F. Stalzer, and D. T. Lefferts, Amer. Ind. Hyg. Ass. J.. 29, 79 (1968).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
·
2227
Table I. Experimental Equipment Perkin-Elmer Model 403 atomic Spectrophotometer absorption spectrophotometer Mode Grating Wavelength Light source Lamp current Slit width Absorption cell
Integrator Recorder
Mercury monitor for temperature study
Absorption Ultraviolet 253.65 nm Mercury hollow cathode lamp 6 mA 1 mm (0.7 ,m spectral band pass) 20 cm, 3.3-cm i.d1., absorption cell with fused silica windows Infotronics CRS-200 automatic digital integrator Leeds and Northup Speedomax W potentiometric recorder Laboratory Data Control Mercury Monitor with 30-cm absorption cell
The sensitivity and. ease of operation of the flameless atomic absorption method utilizing the 253.7-nm elemental mercury line is well known (4-6). It has been applied to the analysis of a variety of materials for mercury (612) However, it is subject to interferences from compounds which absorb or scatter the 253.7-nm radiation (6, 13) To eliminate these interferences, a collection medium which is specific for elemental mercury should be used. The use of gold and silver wire and foil as selective mercury collectors is well documented (1, 2, 8, 10-12, 14, 15). Corte et al. (16) have successfully used silver gauze for ambient air mercury collection for sampling times of less than 24 hours. However, we have found that the capacity for retaining mercury with convenient amounts of these amalgamators is too low for use in high (10 /ng/m3) mercury areas for a 24-hour sampling period even at low flow rates. Therefore, our present system utilizes silver wool as an easily obtainable, very selective, and high capacity collecting agent for elemental mercury. The analysis system employs a standard atomic absorption spectrophotometer in the flameless mode at 253.7 nm coupled with a digital integrator to quantitate the collected elemental mercury which is liberated from the silver wool collector by heating in an analytical gas train. Analysis time is less than five minutes per sample. The capacity of silver wool, gold wire, and silver mesh for amalgamation of mercury has been determined. The effect of temperature upon the release of mercury collected on silver wool was also investigated. A comparison of absorption peak height measurements and digital integrator absorption-time areas was made; and the effect of each type of measurement upon precision, useful range of the calibration curve, and sensitivity of the method was deter.
.
(4) N. S. Poluektov, R. A. Vitkun, and Y. V. Zelyukova, Zh. Anal. Khlm.. 19, 873 (1964). (5) W. R. Hatch and W. L. Ott, Anal. Chem.. 40, 2085 (1968). (6) D. C. Manning, At. Absorption Newslett.. 9, 97 (1970). (7) I. Okuno, R. A. Wilson, and R. E. White. J. Ass. Offic. Anal. Chem.. 55, 96 (1972). (8) J. V. O’Gormon, N. H. Suhr, and P. L. Walker, Jr., Appl. Spectrosc.. 26, 44 (1972). (9) M. J. Fishman, Anal. Chem., 42, 1462 (1970). (10) O. I. Joensuu, Appl. Spectrosc., 25, 526 (1971). (11) G. W. Kalb, At. Absorption Newslett., 9, 84 (1970), (12) V. I. Muscat, T. J. Vickers, and Anders Andren, Anal. Chem., 44,
218 (1972). (13) (14) (15) (16)
R. L.
Windham, Anal. Chem.. 44, 1334 (1972).
W. W. Vaughn, U.S. Geol. Survey Circular. 540 (1967). O. I. Joensuu, Science. 172, 1027 (1971). G. Corte, L. Dubois, R. S. Thomas, and J. L. Monkman, Division of
Water, Air and Waste, 164th National Meeting, ACS, New York, N.Y., August 1972.
2228
·
mined. The effect of the carrier gas flow rate in the analytical system on the detection limit and precision of the method was studied and optimized. The syringe calibration technique involving mercury saturated air was carefully investigated. Interferences from compounds which may be present in ambient air—e.g., sulfur dioxide, nitrogen dioxide, and hydrogen sulfide—were also studied, as was the collection of dimethylmercury on silver wool.
EXPERIMENTAL Collectors. Mercury is amalgamated on a collector which is a 100-mm long, 5-mm i.d., borosilicate glass tube equipped with ball joints on the ends and packed snugly with 1 or 2 grams of cleaned silver wool. The silver wool is Fisher micro-analysis grade and is initially cleaned by placing it in a furnace at 800 °C for 2 hours. In order to release the collected mercury, the collector is permanently wrapped with exactly 100 cm of 22 gauge Nichrome heating wire. Complete release of mercury from the silver wool is achieved by applying 24 V from a variable transformer across the wire for 30 seconds which yields a temperature of 400 °C. After one 30-second heating period, the collector is mercury free and ready for future use. Collector Capacity. The approximate capacity of 5-mil gold wire, 40-mesh silver gauze, and Fisher micro-analysis silver wool for analgamation of gaseous elemental mercury was determined in the Perkin-Elmer 403 analytical system at a carrier gas flow rate of 200 cm3/min. Repetitive injections of 260 to 660 ng mercury were made into the collecting agent at room temperature until initial breakthrough of elemental mercury was detected. This method introduces the mercury at a much more rapid rate than would occur during actual sampling, and the capacity found is a lower limit to the actual capacity. The types of collectors employed for this study were: a) 1.2 g of 5-mil gold wire packed in a 5-mm i.d. glass tube; b) 0.5 or 1.0 g of silver wool packed in an 8-mm i.d. tube; c) 11.0 g of tightly rolled, 40-mesh silver gauze in a 1.1-cm i.d. tube. Deamalgamation Temperature. A Laboratory Data Control Mercury Monitor (LDC) was used to investigate the temperatures, over the range of 20 to 390 °C, at which amalgamated mercury is initially and totally released from silver wool. The LDC, equipped with a 30-cm absorption cell, was used instead of the Perkin-Elmer 403 Atomic Absorption Spectrophotometer because of the detector’s greater sensitivity. The rest of the analytical system remained unchanged except for the use of a semimicro combustion furnace for heating and a calibrated thermometer which was inserted into the silver wool. Collectors (9-mm i.d. tubes charged with 2 grams of silver wool) were initially loaded at 20 °C with either 55 ng or 66 ng mercury vapor. Carrier gas flow was 200 cm3/min for loading, heating, and analysis. Mercury released from the heated silver wool collector was collected downstream on a second 1-gram silver wool collector at room temperature. The amount of mercury on this second collector was determined after each 5 to 20 °C rise in temperature of the heated silver wool. Analytical System. The analytical system consists basically of a gas manifold into which the silver wool collectors are introduced, a heating system to liberate the amalgamated mercury into the carrier gas, and an optical absorption detector (the P-E 403 Spectrophotometer) to determine the amount of mercury vapor evolved from the heated silver wool. A list of the experimental equipment is given in Table I. The gas train—which is constructed of Vi-inch i.d. glass, Tygon tubing, and Kel-F coated valves—is shown schematically in Figure 1. Very short pieces of Tygon tubing were used solely for connections between the glass tubing and valves since we have found that the tubing adsorbs small amounts of mercury. Rubber and Teflon tubing adsorb larger amounts and should not be used. Flow rates through the manifold of 100 to 1000 cm3/min are produced by a pump behind the absorption cell and are monitored by Fischer-Porter flowmeters. The room air flowing through the system is pre-filtered through activated charcoal and silver wool to remove ambient mercury and/or organic materials. The 20-cm, fused silica, absorption cell is heated to approximately 90 °C with a heating tape to prevent condensation of mercury in the cell. A large activated charcoal trap before the pump collects all of the mercury introduced into the system. Calibration. The injection of known amounts of air saturated with mercury vapor onto a collector in the analytical train is used to standardize the detection system. The known amount of mer-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
VARIABLE TRANSFORMER
Figure 1. Schematic diagram of analytical gas train
cury vapor must be collected onto silver wool and then released by heat in the analytical train since mercury vapor injected directly into the analytical train gives lower results. The carrier gas flow rate is 200 cm3/min. Three 1-liter borosilicate glass bottles containing enough Reagent Grade mercury to cover the flask bottoms and equipped with serum caps are used as standardization reservoirs. They are maintained at 20.0 ± 0.1 °C in a constant temperature bath equipped with an NBS certified thermometer. The amount of mercury vapor contained in 1.0 cm3 air at 20 °C and 1 atmosphere pressure at equilibrium over the liquid is 13.19 ng (See the Appendix for the table of mercury vapor density us. temperature). By using gas-tight syringes with volumes of 0.02 to 100 cm3, 0.26 to 1319 ng can be introduced into the analytical system through the injection port. The accuracy of the 0.5 to 50 cm3 gas-tight syringes was determined by air displacement. A thin, single soap bubble was formed near the top of an otherwise empty 10.0-ml microburet or standard 50-ml buret in a vertical position. A serum cap was placed on the top of the buret, the buret laid horizontally, and the stopcock opened to equalize the pressure. The syringe needle was inserted through the serum cap, and the position of the bubble recorded. A known volume of air was slowly injected from the syringe into the buret, and the displacement of the bubble noted. The average of five trials per volume was used to calculate the per cent deviation relative to the average buret volume. Inter-comparison of a fixed volume obtained from different syringes, with total volumes of 0.25 to 100 cm3, was made by injecting the mercury vapor from each syringe into the P-E 403 or LDC detection system and recording the integrated absorption-time area on a digital integrator. Calculation of the relative standard deviation (RSD) for a particular syringe volume was made relative to the mean of all trials for that particular volume for all syringes tested. Care must be taken when using the mercury vapor pressure injection method to ensure reproducible and accurate results. The sample should be withdrawn slowly, expelled into the reservoir, and withdrawn slowly again to aid mixing. Rapid withdrawals will produce erroneous results. The temperature of the syringe must be equal to or greater than the temperature of the mercury saturated air in the reservoir to prevent supersaturation of the withdrawn air and possible condensation of mercury in the syringe. After injection into the analytical system, the partial vacuum in the reservoir should be relieved by injecting the volume withdrawn back into the reservoir. By using a large reservoir volume, the ratio of the volume withdrawn to total volume is kept at a small fraction. For withdrawals greater than 10% of the total volume of the reservoir, 10 to 20 minutes are required to establish equilibrium between the liquid and gaseous mercury again. Maintaining a clean liquid mercury surface facilitates rapid
equilibration.
A complete calibration curve of seven standards over the range of 0.5 to 594 ng was run in duplicate at the beginning of each analysis period, and standards were run intermittently throughout the day to check the system. Procedure. Mercury in ambient air is collected by pulling air through cleaned collectors, containing 1 or 2 grams of silver wool and connected in series, at a known rate for a fixed length of time. For field sampling purposes, a flow rate of 100 cm3/min for 24 hours is anticipated. The flow rate, collection time, and number of collectors can be varied to suit the expected mercury level. After use, the collector is disengaged from the pump and capped to prevent contamination. After calibration of the analytical system using a similar collector, the collector to be analyzed is clamped into the system by ball-joint clamps, the carrier gas flow is adjusted to 200 cm3/min, and the collector is heated for 30 seconds at 24 V producing a temperature of approximately 400 °C.
Table 11. Operating Parameters of the Infotronics CRS-200 Automatic Digital Integrator Track rate Mark Recorder response Min. peak Recorder attenuator Max. peak Peak sensor gain Mode Atomic absorption connection
30yuV/min On
Linear 10 X1 (most sensitive)
300 2
Automatic P-E 403 recorder output
The resulting absorption-time area signal obtained from the P-E 403 recorder output is recorded on the digital integrator, and the amount of mercury is determined from the calibration curve. The collector is allowed to cool, removed from the analytical system, and capped for future use. The operating parameters of the digital integrator are summarized in Table II. The entire analysis process takes less than 5 minutes per sample at 200 cm3/min carrier gas flow rate using the integrator. The absorption cell bypass, shown in Figure 1, is convenient when collectors are being cleaned, and the amount of mercury released does not need to be monitored. Interferences. Interferences from NO2, SO2, H2S, and dimethylmercury were investigated by Battelle-Columbus Laboratories (17) under contract for this group. Calibrated permeation tubes of the NO2, S02, and H2S potential interferers and of elemental mercury were placed in constant temperature water jackets held at 26 °C with a 100 cm3/min flow of nitrogen passing over the permeation tubes and into the 1-gram silver wool collector. The mercury and potential interferer wore mixed before contacting the silver wool. By varying the flow rate and/or collection time, levels of mercury collected were varied from 11 to 161 ng. The concentrations of the H2S were varied from 13 to 650 gg/m3; the SO2 from 37 to 3700 ^g/m3; and the NO2 from 25 to 100 ^g/m3. After simultaneous exposure to the mercury and potential interferer, the collector was removed from the system and analyzed for collected mercury. Since a calibrated dimethylmercury permeation tube was not available, a standard solution of dimethylmercury in ethanol (1.98 jig Hg/ml) was used to evaluate the possible collection of this compound on silver wool. Volumes of this standard solution containing 99 ng of mercury were evaporated in a nitrogen carrier gas flowing at 100 cm3/min, and the resulting dimethylmercury in the carrier gas was passed over a 1-gram silver wool collector. The exposed collector was then analyzed for elemental mercury.
RESULTS AND DISCUSSION Collector Capacity. There
are no
capacity data in the lit-
erature for the various types of collectors, gold or silver. Therefore, gold wire, silver gauze, and silver wool were tested for their mercury retention capacities by repetitive injections of 260 to 660 ng of mercury onto the collectors at 200 cm3/min until breakthrough occurred. Five-mil diameter gold wire (1.2 g in 5-mm i.d. tube) showed a capacity of 434 ng of mercury per gram of gold wire. The 40-mesh silver gauze (11 g in 11-mm i.d. tube) retained (17) D. L. Chase, D. L. Sgontz, E. R. Blosser, and W. M. Henry, Battelle Columbus Laboratories, ERA Contract No. EHSD 71-32, Environmental Protection Agency, June 1972.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
·
2229
Table III. Precision of Calibration Curve Data
on
One and
on
Three Different Silver Wool Collectors0 Three different collectors6
One collector6 Av area, Hg concn,ng
0.53 6.59 13.2 132 224 264 330 396 462 594
sec X
µ-
10“3
1.17 24.86 46.63 480.8 774.6 868.3 1049 1207 1384 1631
RSD ,d % 11
1.5
0.96 2.9 2.1
1.4 2.2 1.2 1.2 0.49
Av peak height, 0.095-in. units
0.25 5.25 10.20 100.0 158.5 176.0 205.9 235.0 259.8 290.0
RSD,d % .
.
Av area, µ sec X 10"3
RSD,d %
Av peak height, 0.095In. units
RSD,d %
,e
1.1 1.1
2.6
23.79 493.5
5.0
5.0
4.3
1.7
102.3
1.2
1.8
255.2
2.4
0.77 0.85 2.6 1.5 2.7 2.2
1386
“ Data obtained over several days by syringe Injection of mercury saturated air onto 1- or 2-gram silver wool collectors at 200 cm3/min and subsequent heating. Peak heights and areas were measured simultaneously. All areas were corrected for a 245 µ -sec noise level per minute of integration time. 6 All data obtained on the same 1-gram silver wool collector, 6 Data obtained at different times from three different collectors: a new 1-gram silver wool collector, an old 1-gram silver wool collector, a new 2-gram silver wool collector. d Standard deviation calculated from 5 (for 1 collector) to 19 (for 3 collectors) data points by range method of reference (19). e Limit of detection for peak height method.
TEMPERATURE, °C
Figure 2. Effect of temperature on mercury release from silver wool at 200 cm3/min flow rate. A 2-gram silver wool collector was initially loaded with 55.3 ng mercury 700 ng of mercury per gram of silver gauze. The silver wool (0.5 or 1 g in 8-mm i.d. tube) retained three to four µg mercury per gram silver wool before breakthrough. All of these collectors were cleaned before usage by heating at 800 °C for 2 hours. The capacity of the various collectors is dependent on the rate of mercury of packing of the collecting collection and the manner agent. Lower concentrations of mercury, as found in ambient air, will have greater contact times with the collecting agent than did the mercury from the high level injections used in the laboratory. Therefore, the cited capacities are lower limits to the true values. It is very important that the collecting agents be cleaned before use by heating at 800 °C for 2 hours. Otherwise, very low capacities will be found. Because of the expected wide range of mercury levels in ambient air and the higher capacity of the silver wool, all collectors to be used for mercury analysis were constructed with silver wool. Newly made or extensively used collectors containing 1 or 2 grams of silver wool have shown no noticeable variation in capacity or release characteristics. The blank values found on previously cleaned 1-gram collectors that were capped and stored for up to 1 week did not exceed 0.5 ng mercury. Release Temperature. The release temperature for mercury amalgamated on silver wool was determined by slowly heating a 2-gram silver wool collector held in a microfurnace. The collector was previously loaded with 55 or 66 ng mercury; and the released mercury was collected, at a 200 cm3/min flow rate, on a 1-gram silver wool collector downstream. The 2-gram collector was held at each temperature for approximately 1 minute. Figure 2 shows the
quantitatively
2230
.
Figure 3. Calibration curve by peak height and area measurements at 200 cm3/min flow rate. Data obtained on single 1gram collector and mean of 3 collectors are indistinguishable on this graph On single one-gram collector:
,
area,
, peak height
percentage of the loaded amount of mercury which was released at temperatures from 20 to 390 °C. The curves for 55 or 66 ng loaded mercury were essentially identical. At about 80 °C, 1 to 2% of the loaded mercury is released from the collector; and at 220 °C, 96% of the loaded mercury is released. Total mercury recovery was 99 to 114% for the experiments using 55 and 66 ng mercury. Syringe Calibration. The accuracy of the 0.5- to 50cm3 gas-tight syringes, as determined by air displacement into micro- or regular burets, was found to be better than 1.3% RSD, relative to the mean buret volume for that particular syringe volume. The precision for 0.25- to 100cm3 gas-tight syringes, as found by integrated area measurements on the LDC detector and the P-E 403, was better than 1.2% RSD for volumes of 50 cm3 or less and better than 2.2% for volumes of 50 to 100 cm3. Retention of mercury by the syringes, over the range of 0.66 to 660 ng, was less than 1% of the injected amount. Syringes should be conditioned each day before use by filling with mercury saturated air and letting stand for a few minutes before use in calibrating the analytical system. Use of a syringe to measure volumes at the limits of the syringe range will give poor accuracy and precision.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
360
Table IV. Effects of Flow Rate upon Precision and Detection Limit0 Flow rate,
cm3/min 100
200
500
1000
Detection limit,c ng
Hg concn, ng
Area RSD,6 %
13.2 132 594 13.2 132 594
2.10 2.83 0.63
0.2
0.81
0.3
13.2 132 594 13.2 132 594
1.56 0.49 1.84 0.69 0.94 0.88 0.69 0.82
0.3
0.9
“Determined on 1-gram silver wool collector by syringe injection of mercury onto collector and subsequent heating of collector. 6 Standard deviations calculated from five trials using the range method of reference 19. c Concentration corresponding to signal at 2 standard deviations above average background.
Analytical Curve. The majority of papers describing flameless atomic absorption methods for determining mercury use the signal peak height as a measure of the total mercury in the analyzed sample. Kalb (11) suggested the use of integrated areas rather than peak heights, unless the variables affecting the shape of the absorptiontime curves were very carefully controlled. Thomas et al. (18) used a digital integrator in a method to measure mercury in fish, but they did not cite data for both peak height and area measurements. In Thomas’ study, it was found that the use of peak heights did not give a faithful representation of the total mercury in a sample. In our study both absorbance-time areas and peak absorbances were measured simultaneously for replicate mercury injections on a single 1-gram silver wool collector and on three different collectors—an old 1-gram, a new 1-gram, and a new 2-gram collector. The results of this comparison with regard to precision and calibration curve shapes are given in Table III and Figure 3. All of these data were obtained at a flow rate of 200 cm3/min over the range of 0.5 to 594 ng mercury. As can be seen in Table III, the precision of both methods of measurement on the same collector are essentially identical (a t-test at the 90% confidence level shows no difference) over the range of 6.6 to 396 ng mercury. However, the integrated area was more reproducible at higher mercury concentrations—e.g., 594 ng integrated area had 0.49%; RSD and peak height had 2.2% RSD. In the case of the data obtained on three different collectors, the same conclusions can be drawn. However, at mercury levels of 6 ng or less, better precision can be attained by using the area measurement and by calibrating on one collector. If highly precise data are desired, it is advisable to calibrate on the same collector which was used for field collection. An alternative method is the distillation of the collected mercury onto a fixed collector in the analytical train. All calibration could then be done on the fixed collector. The differences between the precision of the peak height and area measurements have been minimized by our controlled construction and heating techniques. These comparisons would not hold for less carefully controlled systems where it would be expected that the area method would be much superior (18). Another important factor in the analysis is the sensitivity—t.e., signal per unit amount mercury—of the analyti(18) R. J. Thomas, R. A. Hangstrom, and E. J. Kuchar, Anal. Chem.. 44, 512 (1972).
Figure 4. Effect of flow rate upon calibration measurements on 1 -gram silver wool collector Flow rates:
,
100 cm3/min;
,
200 cm3/min;
,
curve
by area
500 cm3/min; V,
1000 cm3/min
Figure 3 illustrates an analytical curve obtained single 1-gram collector (data of Table III) at a flow rate of 200 cm3/min using both peak height and area measurements. The peak heights, in units of 0.095 inch, have been scaled by multiplying by 4,823 to present both on the same graph. All area measurements were curves corrected for an average 245 µ -sec per minute noise level on the integrator. It is readily apparent that the area calibration curve is superior to the peak height curve in both sensitivity and useful upper limit. For mercury amounts greater than 594 ng, both curves show a low sensitivity. Because of the better precision at high mercury levels, better detection limits, higher sensitivity, and wider useful range, the area method was chosen for all further tests. If a digital integrator is not available to the analyst, integration by cutting and weighing of the recorded curves
cal
curve.
on
a
will serve as a substitute. Effect of Flow Rate. The rate at which the carrier gas and the vaporized mercury flow through the analysis system into the absorption cell and the precise maintenance of this flow rate is extremely important. Therefore, the effects of flow rates of 100 to 1000 cm3/min on a 1-gram collector were studied. As shown in Figure 4 and Table IV, changes in the flow, rate affect the precision, sensitivity, and detection limit very markedly. With a low flow rate, the average mercury concentration in the absorption cell is higher and results in a lower detection limit, but a longer analysis time is required. For a flow rate of 100 cm3/min, a detection limit of 0.2 ng was found, while a flow rate of 1000 cm3/min gave a detection limit of 0.9 ng. Although the integrated area results are slightly more reproducible at the higher flow rate of 1000 cm3/min for most mercury concentrations, the slope of the calibration is greatly reduced (see Figure 4). A flow rate of 200 curve cm3/min was chosen as a compromise due to its very good
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
·
2231
Table V. Interference Effects of SO2, NO2, and H2S Silver Wool Collectors0
on
Table VI. Statistical Evaluation of Interference Effects on Silver Wool
Potential in-
Pooled
terferer
Potential interferer
concn,
µ /m3
Hg added, ng
None
120
37
130
43
17
16
13
6
100
N02
50
25
1
6
5
160.5 64.2
3
88.1
1 1
8 1
2 5 1 1 1 1
4 5
4.3d
4.6 2.0 5.3 12.5 5.3 11.2
1
9 1 1
2 1
4 3 3
Pooled avc 106.28
5.0
7.7 0.1
9.4
6.7 7.4 6.7 0.8 2.9 0.1
6.2d 1.8
10.6
11.2
9.2 '
11.1 0.1
9.2d
0
Mercury and interferer, at the concentration specified, were premixed before the mixture was passed through 1-gram silver wool collectors at 100 cm3/min. 6 Standard deviations were calculated by the range method of reference 19. c The pooled average Hg recovery was calculated by summing the weighted averages and dividing by the sum of the weights. The weighting factors were /V¡/s¡2, where N¡ is the number of data points used to compute the /th average Hg recovery % and s¡ is the standard deivation of the ith set of points about their mean. No single Hg recovery data points were included In this pooled average. d The pooled standard deviations, Sp, were calculated by the formula. S2P
-
-
=^3(/V,:
1)S,i2j/(N
K). where S¡ is the standard deviation of the
/th set of data, N Is the sum
of all N¡, and K is the number of sets
pooled.
(19) R. B. Dean and W. J. Dixon, Anal. Chem., 23, 636 (1951).
2232
·
ence6
Caled
1.64 6.33 5.83
1.20 5.12 2.81
InterCritical ference
2.726 2.737 2.797
No Yes Yes
6
Table VII. Precision of Analysis of Field Collected Samples0
Mean Hg found,
7.3d
112.1
100.0 104.7 104.9 100.6 83.9 107.8 104.7 118.0 101.0 106.3
f6
Differ-
See Table V. Pooled average Hg recovery for no interferer minus pooled average Hg recovery for interferer. c Student f-test for difference between pooled means. The critical t value listed is at the 99% confidence level for the appropriate number of degrees of freedom.
6.6
84.8 90.3 Pooled avc 94.12 3
N02 0
3.1
8 3
1
H2S
recov-
Av calibration curve6
98.81 97.7 114.1
32.1
107.0 64.0 43.0 32.0 161.0 118.0 64.0 43.0 32.0 64.0 32.0
3.3 3.7
Pooled av6 100.45 4 97.8 5 96.3 9 101.4 1 81.3 2 86.0 9 95.1 1 104.7 7 101.3 1 109.1 1 99.1 4 102.8 1 96.6 8 110.2
105.0 104.7 113.7 100.0 100.0 104.4 97.2 98.4 100.0 100.0 89.3 90.0 89.7
32.1
4.3 7.3 6.2 9.2
5.1
8
43.0 32.0 161.0 64.0 32.0 161.0 64.0 32.0 107.0 64.0 43.0 32.0 160.5 64.2
650
100.45 98.81 94.12 106.28
covery6
104.7 99.7 98.0
Pooled avc H2S
None S02
av re-
covery, %
64.2 160.5
75.0 43.0 32.0 43.0 32.0 11.0 107.0 64.0 59.0 32.0
ery0, %
Av Hg re-
runs
10
32.1
180
terferer
Pooled std dev6
No. of
32.1
160.5 64.2
3700
S02
av Hg
Std dev of
Potential in-
Calibration of actual collector6 Mean Hg found,
Date
ng/m3
RSD,d %
ng/m3
RSD,d %
10/24 10/25
26.7 31.4
10.2 9.4
25.4 26.2
9.5 6.5
0 Samples were collected in parallel using four pairs of a 1-gram and 2-gram silver wool collector In series for 24 hours. The flow rate was about 200 cm3/min and was controlled by the use of calibrated glass critical orifices. Analysis was performed 16 days after collection. Cleaned collectors before use had 0.5 ng Hg/m3 or less. 6 Data calculated from a calibration curve obtained on a 1-gram silver wool collector which was not used for collection. A linear curve for standards from 0.61 to 10.6 ng fit to 3.8% relative standard error of estimate. 6 Data calculated by Individual calibration of the actual collectors (16 total) used in the field. d Standard deviations calculated by the range method of reference 19 from the four sets of data.
a
sensitivity, reproducibility, and detection limit. Samples of even high mercury concentrations (1319 ng) can be run in 5 minutes or less at this flow rate, whereas analyses performed at 100 cm3/min require more than 5 minutes for 131 ng. This time factor is important when a large number of analyses is anticipated. Detection Limit. The detection limits, defined here as the concentration corresponding to the signal two standard deviations above the average noise level for the analysis time, were determined by the area method at flow rates of 100 to 1000 cm3/min. These are listed in Table IV. For our integrator setting, the mean noise level, determined at random over 6 days, was 245 µ -sec per minute. At 200 cm3/min, our selected flow rate, the detection limit was 0.3 ng. No particular efforts were made to decrease the detection limits by optimizing the atomic absorption parameters. It is quite possible that this would lower the detection limits. Interferences. The effects (17) of SO2, NO2, and H2S mixed with mercury at various concentrations and collected at 100 cm3/min carrier gas flow on a 1-gram silver wool collector are shown in Tables V and VI. As can be seen from Table VI, no statistically significant interference at the 99% confidence level was found from SO2 over the range of 37 to 3700 ^g/m3. A barely significant positive interference, based on the 0.01 difference between the calculated and critical t values, was found for NO2 over the range of 25 to 100 µg/m3. This range includes the average level of 70 µg/m3 for ambient air. However, H2S gave a significant negative interference of about 6% at levels of 13 to 650 µg/m3. This range exceeds the ambient level of H2S of less than 10 µg/m3. Tests of the possible collection of dimethylmercury on the silver wool by evaporating several portions of dimethylmercury in ethyl alcohol onto a collector gave less than 1-ng signal for 99-ng elemental
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
mercury portions. Other workers (20) report that dimethylmercury vapor collects on silver and decomposes to yield elemental mercury upon heating the silver. Ambient levels of dimethylmercury are so low that this effect is negli-
gible. Very high levels (20-¿tg injections) of chlorine attack the silver wool collectors (21). However, a basic scrubber before the collector will eliminate this problem. Moreover, the ambient level of chlorine is certainly less than 10 µg/ m3. Therefore, this interference and those interferences (±6%) due to H2S and NO2 are not considered to be serious for ambient air measurements since high levels of these interferers should not be encountered. Analysis of Field Samples. Elemental mercury was collected from ambient air near our laboratories using 1and 2-gram collectors in series at a flow rate of approximately 200 cm3/min for 24 hours. Quadruplicate sets of collectors were used in parallel on two different days. The analysis of the samples was performed by two methods. A calibration curve determined on a similar 1-gram silver wool collector was used for all analyzed collectors. Then the mercury levels were more closely defined by obtaining two calibration points on the actual field collectors. The results which include imprecision contributions from the analysis of both collectors and the collection procedure are given in Table VII. The relative standard deviation (RSD) for the method employing an average calibration curve was 9.4 to 10.2%. The RSD obtained by calibrating on the individual collectors was 6.5 to 9.5%. There appears to be no statistically significant difference between the two methods of analysis based upon a F-test at the 99% confidence level.
CONCLUSIONS The method for analyzing elemental mercury in ambient air presented here is selective, precise, rapid, useable over a wide range of mercury levels, and effectively interference-free (6% maximum) from the effects of ambient SO2, NO2, H2S, and chlorine. The use of silver wool collectors gives a simple, inexpensive, high capacity, and selective medium for field collection of elemental mercury. The analysis scheme is simple, rapid, and employs standard atomic absorption equipment. For a very wide range and/or low levels of expected mercury, the area rather than the peak height method should be used. No solutions are used for calibration. However, solutions could easily be analyzed by reduction and aeration of the mercury in the solution and subsequent collection on silver wool (12). (20) G. Corte and J. L. Monkman, Air Pollution Control Directorate, Environment Canada, Ottawa, Canada, personal communication, 1973. (21) T. M. Spittler, R. J. Thompson, and D. R. Scott, Division of Water, Air and Waste, 165th National Meeting, ACS, Dallas, Texas, April 1973.
Table VIII. Elemental Mercury Vapor Density at 10 to 30 °C and 1 Atmosphere Pressure /,
°c 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5
Vapor density, ng/cm3
5,57 5.82 6.09 6.36 6.65 6.95 7.26 7.58 7.92 8.27 8.63 9.01 9.41
9.82 10.25 10.69 11.15 11.63 12.13 12.65 13.19 13.75
t,
°C
21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0
Vapor density,
ng/cm3 14.33 14.93 15.56 16.21 16.89 17.59 18.32 19.07 19.86
20.67 21.52 22.39 23.30 24.24 25.22 26.23 27.28 28.36 29.49
APPENDIX The vapor densities of elemental mercury in saturated air at one atmosphere pressure and various temperatures were calculated by using the ideal gas law and mercury vapor pressure (22) fitted to a linear least squares equation in log vp vs. 1/T. The vapor densities, ng/cm3, are tabulated in Table VIII for temperatures from 10 to 30 °C.
ACKNOWLEDGMENT We wish to thank J. L. Monkman and G. Corte of the Air Pollution Control Division, Environment Canada, for discussing their elemental mercury analysis using silver gauze with us during the early stages of the present work. We also wish to thank Teri Teeri, Louis Pranger, and Larry Holboke for assistance with various phases of
the study. Received for review April 9, 1973. Accepted May 23, 1973. Presented at the 165th National Meeting, American Chemical Society, Dallas, Texas, April 1973. Reference to brand names of equipment or chemical sources does not constitute endorsement by the Environmental Protection Agency. (22) “Handbook of Chemistry and Physics," 47th ed., R. C. Weast, Ed., The Chemical Rubber Co., Cleveland, Ohio, 1966-1967, p D108.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
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