Improvements in Cold Vapor Atomic Absorption Determination of Mercury J. E. Hawley and J. D. Ingle, Jr.' Department of Chemistry, Oregon State University, Corvallis, Ore. 9733 1
Modifications to the normal apparatus used for the cold vapor atomic absorption determination of mercury have been made by reducing the dead volume of the apparatus, by increasing the efficiency of diffusion of elemental mercury into the carrier gas, and by optimizing the instrumental parameters. The time of analysis, the sample volume, and the detection limit have been greatly reduced. For a 1-ml sample, the concentration detection limit is 1 ppt of Hg(ll). The relative standard deviation for 0.05-10 ppb of Hg(ll) is approximately 3 %. At concentrations below 0.05 ppb Hg(ii), imprecision is due mainly to the fluctuations from the radiation source; and, at concentrations above 0.05 ppb Hg( ll), sampling imprecision is limiting. To prevent mercury losses, an oxidizing agent and acid should be used as a preservative and, to prevent contamination, ail glassware must be adequately cleaned with the oxidizing solution.
Mercury in the environment is of great current interest because of its toxicological effect on plant and animal life. Sources of mercury are man-made, such as industrial wastes, pesticides, and fungicides, or natural, such as mineral deposits. T o pinpoint the sources of mercury contamination and to evaluate the levels of mercury contamination in a host of materials, extensive research on mercury determination has been conducted as evidenced by a number of review articles (1-3). In particular, the cold vapor atomic absorption method has received the greatest attention because this technique normally provides detection limits on the order of 1.0 p p b and is relatively simple and free from most interferences. T h e best detection limit reported ( 4 ) for this technique is about 0.02 p p b Hg(I1). Determination of residual mercury in natural water is a particular problem because mercury levels in natural water are often 0.03-0.06 p p b or lower and tedious preconcentration procedures are necessary (5-9). These facts indicate the need for a more sensitive instrument for the direct determination of mercury in natural waters. A critical study of the cold vapor method for atomic absorption determination of mercury was made in order to improve the instrumentation, t o lower the detection limit for mercury, and to improve the precision of analysis a t sub-ppb mercury concentrations. In addition, a more fundamental understanding of variables t h a t affect the technique and of the factors t h a t affect the precision and accuracy of analysis was desired.
EXPERIMENTAL Solution and Glassware Preparation. The stock solutions were prepared from reagent grade chemicals and distilled water as indicated in Table I. The SnC12 solution was bubbled with nitrogen to remove mercury contamination. All standards contained 1% (v/v) "03 (concd) and 0.002% ( w i v) KMn04 as a preservative and were prepared by dilution of the 100-ppm mercury stock solution. They were analyzed within eight hours of preparation.
To whom correspondence should be addressed
Glassware was soaked in a nitric acid-permanganate solution for 24 hours. Afterwards, the glassware was washed with HC1 (concd),
(concd), and rinsed rinsed with distilled water, washed with "03 with distilled water. Instrumentation. The final instrumentation selected is shown in Figure 1 and the specific components are identified in Table 11. The lower half of Figure 1 shows the basic elements of a doublebeam AA instrument with direct linear absorbance readout. Initially, a single-beam system was used. However, the improvement in long-term stability and the reduction of short-term source flicker noise made the double-beam system preferable for convenience and for the best precision and detection limits. The dc lamp power supply for the pen lamp was constructed from a 0-600 voltage source in series with a 20-K ballast resistor and a 20-mA current meter. The lamp was enclosed in a small cardboard box to reduce the effect of air currents on the lamp temperature and, hence, on the lamp radiance. For dc operation, the lamp is warmed up for 30 minutes with the ac transformer and started with a vacuum tester. The polarity of the electrodes should be changed every two weeks to ensure a long lifetime and stability. The voltage amplifier was constructed from an OA, a power supply, and appropriate connectors mounted in a metal box. Rotary switches allow selection of various feedback precision resistors, of feedback capacitors, of different input precision resistors, and, hence, of the voltage gain and RC time constant. The logarithmic ratio amplifier was constructed from a commercial log ratio module, power supply, and connectors mounted in a metal box, and it produces 1 volt per absorbance unit. The beam splitter is a quartz window placed in the center of a sample holder. A small percentage of the source radiation is reflected 90' and passes through a 3mm aperture and a 254-nm interference filter to the reference photomultiplier tube. The upper half of Figure 1 shows the flow system. During analysis, the carrier gas, air, flows from the compressed air tank (breathing quality) and regulators through a T-bore stopcock, the reduction vessel, a drying tube, the absorption cell, another T-bore stopcock, a flowmeter, two mercury oxidizing traps (chromic acid and acid-permanganate solutions in filtering flasks), and finally into the hood. The two T-bore stopcocks can be turned 90' to allow the solution in the reduction vessel to be evacuated into the filter flask without drawing the oxidizing solutions in the traps back into the line. The drying tube is made of Tygon tubing, filled with Mg(C104)2, and sealed with quick-connect ends. The reduction vessel was constructed from a glass sealing tube with a 1-cm porous frit. One end is tapered for connection to 3/le;-in. i.d. tubing and the other end is widened to 15-mm 0.d. to house tightly a No. 20 sleeve type rubber stopper for injection of samples. There is a glass exit tube 2 cm beneath the stopper and 8 cm above the frit. Pointed impressions, about 4 cm from the top, stop bubbles from creeping out the exit tube. The absorption cell was constructed by attaching 1-inch diameter quartz windows with apiezon wax to the ends of glass tubing. Inlet and outlet ports (3/16-in.0.d.) were attached about 1 mm from each end. Different lengths and cell diameters were studied as discussed in a later section. For alignment of the absorption cell, it is mounted with tube holders on vertical translators upon an optical rail secured to a horizontal translation stage. Procedure. The final procedure used for analysis and for optimization is discussed below. Instrumental variables are adjusted to the values noted in Table I11 after optimization of the cell position. Both the sample and reference photomultiplier power supplies are adjusted to produce 10-5-A photoanodic currents. The voltage amplifier gain is adjusted so as t o give a deflection at least half of full scale (1 V) on the recorder. One-tenth of a milliliter of reductant is injected with a 1-ml syringe into the reduction vessel after the carrier gas flow has been initiated through the frit. The bubbling of the reductant removes mercury that may be present. After the reANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
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719
Table I. Solution Preparation
Table 111. Optimal Values for Analysis
Reductant: 1 g SnC1, -t 1 m l HCl(concd) diluted t o 100 in1 [I% (w/v) S ~ C I , ] Oxidant: 0.2 g KMn04 diluted t o 100 m l [0.2% (w/v) m n o ,I Mercury: 0.1354 g HgC1, + 50 m l HNO,(concd) diluted to 1 1. [lo0 ppm Hg(I1) stock solution]
Table 11. Components of Instrumentation Item
1. Sources and power supplies M e r c u r y pen l a m p
Supplier and Model
Ultra- Violet Prod.
- 11sc-1c
2. 3.
4.
5.
6. 7.
a.
AC power supply DC power supply Mercury hollow cathode tube Monochromator Photomultiplier tubes, p o w e r supplies, and holders Sample PMT Power supply Holder Reference PMT Power supply Holder Log r a t i o amplifier Log r a t i o module Power supply Voltage amplifier Operational a m plifier Power supply Strip chart recorder F 1o w m e t e r Flow r e g u l a t o r s
Beam s p l i t t e r Housing Interference filter (254 nm) Quartz window 10. Sleeve type r u b b e r stopper
Ultra-Violet Prod. -SCT- 1 E/M CO. Westinghouse-WL 22847A Heath- EU- 700/E
Value
Gas carrier Volume of reductant Volume of sample Slit width Absorption cell Radiation s o u r c e Lamp current RC t i m e constant Voltage amplifier gaina Photoanodic current Photomultiplier supply voltagea a
140 ml/min Medium 5-cm long x 12-mm diameter Mg(C104), Air 0.1 m l 1.0 m l 1000 p m 20-cm long x 2-mm i.d. 60-cm long x 2-mm i.d. Hg pen lamp (dc) 9-10 mA 0.32 sec 1-100 10-5 A 500-600 V
Adjusted as described in procedure.
RESSED
I
RCA- 1P2 8 Heath- EU-42A Heath- EU- 701- 93 HTV-R166 s o l a r blind Keithley- 84489 McKee-Pedersen-MP-1021
Teledyne- Philbrick-4361 Analog Devices-915 SUPPLY
RECORDER
SUPPLY
Function Modules-3801 Analog Devices-915 Heath-EU-205-11 Gilmont- F 7260 Matheson-70 Victor-SR- 200
1
t
1
Figure 1. Instrumentation
9.
McKee- Pedersen-MP- 1017 Pomfret R e s . Optics-20-2357- 1 ESCO Optics Prod. VWR-16170-167 ~
corder pen has returned to the base line, 1 ml of Hg(I1) standard is injected with a 1-ml syringe into the reduction vessel. The absorption peak is similar to that previously reported ( I O ) , except the peak width is much smaller. The beginning of the peak is observed about 3 seconds after injection. Within 6 seconds, the peak absorbance IS recorded and, within 20 seconds, the pen has returned to the base line. For measurement of the peak absorbance, a peak base line is interpolated by drawing a line between the base lines before injection and after the peak has passed. The difference between the absorbance at the peak and the absorbance of the interpolated base line is taken as the peak absorbance. The sample then is evacuated and a 1-ml blank is flushed through the frit to eliminate memory effects above 1 ppb Hg(I1) levels. The first stopcock is rapidly alternated between vacuum and carrier gas a few times to remove most of the solution from the frit. The rubber stoppers and the drying tube are replaced after 20 injections and samples can be analyzed at a rate of about 30 per hour. Optimization. General. In this section, the studies of the effects of various instrumental or physical variables on the observed peak height, width, and shape, on reproducibility, and on S/N are discussed. The effects of the variables will be discussed one at a time. 720
Variable
Flow rate Frit g r a d e Drying tube
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4 , APRIL 1975
Unless stated otherwise, a 10-ppb Hg(I1) standard, a 20-cm long, 5-mm diameter absorption cell, and the other settings for the variables listed in Table I11 were employed. The basic analysis scheme described in the previous section was used for all optimization studies. To understand the influence of different variables on the data and to understand why the described system is superior to previous systems in a number of respects, one must first understand the basic processes that occur in analysis. When a Hg(I1) solution is injected into the reduction vessel with the bubbling SnC12 solution, the Hg(I1) ions are reduced to neutral mercury atoms which diffuse from the solution into the carrier gas and are carried through the reduction vessel to the drying tube and finally to the absorption cell. Essentially all of the mercury atoms diffuse from the solution into a given volume of carrier gas. This volume of carrier gas which contains mercury is denoted the plug. Ideally, the plug would be of uniform concentration but in reality the plug-concentration profile appears to have a maximum with respect to time. The diffusion out of the reduction vessel will be controlled by the size and shape of the reduction vessel, the volume of reductant and Hg(I1) solution, the efficiency of aeration, the effective solution surface area during aeration, and the rate constant for diffusion of mercury through a solution of unit depth into the carrier gas. If diffusion of mercury in the carrier gas is important, then the volume of the plug is expected to increase as it moves through the apparatus. The plug of mercury will first be observed when it enters the front of the cell and the maximum value will be observed after the plug reaches the end of the cell and the most concentrated part of the plug is in the cell. The observed peak height or peak absorbance will be dependent on the effective concentration of the plug or of the portion of the plug in the cell. The observed width will depend on the volume of the mercury plug entering the cell and the cell volume. If either volume is reduced, the observed width
I
I
< 120
4
0 90L 24
9 m
0221
0 80
-
~
b 070-
060-
'
I 400
SA0
700
660
Flow r o t e (ml/minI
Figure 2. Relationship of peak parameters to flow rate (e) Half-width(sec), (3 Half-width (mi), and (91 Peak absorbance Internal d i o m e l e r
mml
Figure 4. Effect of cell dimensions on peak absorbance (kj 20-cm length, (Q 60-cm length, and (m) 120-cm length 130-
09CF
i 080L
o
t
z
t
0 200
1 !
.,+ cc
, 04
(12
06
08
I
. ,o
VOLUME 0 1 STbNDARO
[rnl
101
10nqi
Figure 3. Effect of solution volume in reduction vessel on peak absorbance (h) 1.0 mi of SnCi2, (4 0.5 mi of SnCi2, and (j0.1 ml of SnCi2
will be reduced. However, if the cell volume is much smaller than the plug volume, the cell volume should have little effect on the observed width. If the flow rate of carrier gas is doubled but the volume of the plug is unchanged, then the mercury will diffuse out of the solution twice as fast, the velocity of the plug through the system will be twice as fast, and the peak width in time will be reduced. Other authors ( I 1-13) have indicated the importance of maximizing the efficiency of diffusion of mercury into the carrier gas, minimizing the dead volume, and getting the plug completely into the cell to achieve the most concentrated mercury plug. Most researchers ( 4 , 6, 8, 10, .12, 13) have used rather large reduction vessels with inefficient aeration. Recently, Gilbert and Hume (11) reported a reduction vessel similar to the one used in this research. The carrier gas was hubbled through the frit of a Buchner funnel into the solution. The detection limits were much greater than those reported here because of the much larger size of their system. F l o u Rate. The effect of carrier gas flow rate on the peak absorbance, the peak half-width in seconds, and the peak half-width in milliliters (i.e., the product of the half-width in seconds times the flow rate) is shown in Figure 2. The results are similar to those previously reported (10, 11, 13)and also indicate that about 90% of the mercury is in a 20-ml plug of carrier gas and that a flow rate of about 140 mllmin is optimal. Volume Study. The effect of the volume of solution in the reduction vessel on the peak absorbance for a constant absolute mass (10 ng) of mercury was studied. This was done by mixing 0.1 ml of 100 ppb, 0.5 ml of 20 ppb, or 1.0 ml of 10 ppb of a Hg(I1) standard solut,ion with 0.1 ml, 0.5 ml, or 1.0 ml of the SnClz solution. The results shown in Figure 3 are similar to those previously reported
L A -__ . _ ~ 1 . 0 I0 20 30 40 50 C o n c e n t r O l l O n (pph]
Figure 5. Effect of the type of lamp and lamp conditions on calibration curves Pen lamp: (n) 14 mA ac, ( 0 ) 10 mA dc, and ( p ) 6.5 mA dc. Hollow cathode: (4) 10 mA dc and (6 5 mA dc
( I O , 11) and clearly illustrate that the peak absorbance increases with decreasing volume in the reduction vessel because of faster diffusion rate of mercury atoms from the solution into the air stream. One milliliter of standard or sample and 0.1 ml of reductant were chosen for final solution parameters. Smaller volumes of Hg(I1) standards increase the absolute response (Alng) but produce a much poorer concentration response (Alppb) and are more dependent on the reproducibility of sample volumes added. Absorption Cell. Alignment of the radiation source with respect to the slit of the monochromator is made by positioning the lamp in both vertical and horizontal directions to achieve the greatest photoanodic current. The absorption cell is then placed in the tube holders upon the optical rail and is oriented in the X-Y plane with the translators to achieve a maximum photoanotlic current. The peak absorbance was measured for absorption cells which varied in length from 20 to 120 cm and in internal diameters from 18.6 mm to 1 mm, and the results are shown in Figure 4. For a given cell length, the absorbance increases with decreasing cell diameter and approaches a limiting value. The increase in absorbance, as the cell diameter decreases, occurs because the average concentration in the absorption cell increases as the volume of the absorption cell decreases until the cell volume becomes small enough to contain the most concentrated part of the mercury plug. The volume of the ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4 , APRIL 1975
721
Table I V . Percent Losses of M e r c u r y over 24 H o u r s Percent loss O.~%(V/V)
1% ( v i v ) Hh'O3 (concd),
H20, polyethylene Solution
Blank 0.01 ppb 1.0 ppb 10 PPb
polyethylene
-
0.002% ( w / v )
KhtnO4, borosilicate
(concd)
"03
+ 0.0006% (w/v)
KMnO4,
borosilicate
Closed
Open
Closed
Open
closed
closed
... ...
...
...
...
80 28
90 28
60 20
80 20
16 6 10 1
29 29 28 12
...
CHEMICAL A N D PHYSICAL PROBLEMS I%(II) PPt
Sample
T a p water Distilled Twice distilled 1% (v/v) HC1 (concd) Willamette R i v e r
...
...
Table V. W a t e r A n a l y s i s Without preservative
0 0 0 15 0
With preservative
10 3 -1
... 9
plug was estimated (twice the half-width in ml) t o be about 20 ml. The total volume of the cell must be less than about 4 ml to achieve near maximum peak absorbance. For a constant cell diameter of 2 mm, the signal increased 2.6 times from the 20-cm length to the 60-cm length and 3.5 times from the 20-cm length to the 120-cm length. The increase in absorbance with cell length for small diameters can be explained in terms of Beer's law. The absorbance is directly proportional to the length if the analyte concentration is uniform throughout the cell. The negative deviations from Beer's law evident for the 60-cm and 120-cm long cells may be due to the non-uniform mercury plug concentration profile from the 10-ppb Hg(I1) solution. However, at mercury concentrations below 1 ppb, the absorbance with the 60cm length cell is 3 times the absorbance with the 20-cm length cell, thus conforming to Beer's law. For practical purposes, the 120-cm long and 1-mm diameter absorption cells are very difficult to align and it is difficult to maintain the alignment. Therefore, 2-mm diameter absorption cells with 20-cm and 60-cm lengths were chosen as the optimum dimensions. Radiation Sources. A mercury hollow cathode tube and a mercury pen lamp were evaluated as atomic absorption sources with the 2-mm X 20-cm length cell. Figure 5 shows the calibration curves constructed from peak absorbance measurements for three different Hg(I1) concentrations for the two lamps operated under different current conditions. The data indicate that the slope or sensitivity in absorbance units per ppb and the linearity are better for the hollow cathode tube, for lower currents with either lamp, and for dc rather than ac operation with the pen lamp. Below 6.5 mA dc, the pen lamp will not operate. Although the hollow cathode tube provides the greatest slope, a lens was needed to focus and collimate the radiation to produce about one tenth the light level impinge upon the photomultiplier tube as produced with the pen lamp without optics, and the signal from the hollow cathode radiation was shot noise limited. For further analysis, the mercury pen lamp was chosen since it was more intense, since it could be used with longer 2-mm diameter absorption cells, and since it was easier to align. Finally, a 9-10 mA dc current was found to provide the best stability. The negative deviations observed in Figure 5 may be ascribed (13) to differences in the emission line width of the lamps operated under different current conditions and hence at different temperatures. Reduction Vessel. Thr'ee grades of porous frits for the reduction vessel, coarse, medium, and fine, were studied. The peak absorbance obtained with the fine frit was only 2% higher than for the medium frit and 5% higher than for the coarse frit. A medium frit was chosen for all succeeding studies since the fine frit easily clogs and requires the most time to evacuate. A reduction vessel made from a 30-mm diameter medium frit sealing tube with reduced ends gave about a 13% lower peak height than the 10-mm diameter coarse fritted reduction vessel. 722
1% (v/v) HNO3 (concd)
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
S t o r a g e Losses. Mercury losses with polyethylene a n d borosilicate glass containers were studied under different conditions. Feldman (14) presents a n excellent study of t h e preservation of dilute mercury solutions down t o 0.1 p p b a n d reviews previous work in this area. Until this study, all work had been done a t mercury concentrations above 20 ppb. Feldman's a n d our d a t a show that t h e percent mercur y loss with respect to time a n d t h e effectiveness of preservatives are very dependent o n t h e initial mercury concentration. T h e d a t a shown in Table IV for percent loss over a period of 24 hours from time of preparation indicate t h a t t h e 1%(v/v) "03 (concd) 0.002% (w/v) K M n 0 4 preservative is t h e most suitable for mercury storage. For this period of time and especially for longer periods of storage, t h e HN03-KzCr20, preservative suggested by Feldman ( 1 4 ) appears to be superior. M e r c u r y C o n t a m i n a t i o n a n d Blank P r o b l e m s . T h e blank solution will give a positive absorbance reading or mercury solutions will give erroneously high absorbances if t h e blank or mercury solutions pick u p mercury. Mercury contamination results from residue mercury in t h e distilled water, in t h e reagents, from t h e air, or from t h e surfaces of t h e volumetrics, of t h e reduction vessel, or of t h e syringes. T h e cleaning procedures outlined in t h e procedure section should reduce t h e extent of contamination from some of t h e sources. Untreated house-distilled water produces n o blank signal. T h e following solutions were made from house-distilled water: a 1%(v/v) "03 (concd) solution, a 0.002% (w/v) K M n 0 4 solution, and a 1% (v/v) "03 (concd) 0.002% (w/v) K M n 0 4 solution. T h e absorbances of t h e above three solutions were measured. Only t h e last solution gave a measurable blank value and, after diluting this solution 1:2 with house-distilled water, t h e blank value remained constant. Therefore, it was concluded that in t h e strong oxidizing environment of t h e acid-permanganate preservative solution, t h e mercury was released from organomercury compounds, bacteria, or particulate matter in t h e distilled water. Finally, double distilled water in t h e preservative solution exhibited t h e same characteristics as stated above; however, t h e blank value was reduced by about one third.
+
+
RESULTS For t h e 20-cm cell, t h e slope of t h e AA calibration curve is 0.0219 ppb-1 and is linear u p t o about 10 p p b of Hg(I1). At 50 p p b Hg(II), t h e slope of t h e calibration curve is about a factor of three less. For t h e 60-cm cell, t h e slope of t h e calibration curve is 0.0638 ppb-' a n d is linear u p to aboilt 3 p p b Hg(I1). As illustrated by Figure 5 , serious negative deviations occur for concentrations which yield absorbances above 0.5. T o analyze solutions of higher Hg(I1) concentrations, i t would be desirable either t o use t h e pen lamp a t 6.5
mA dc or the hollow cathode t u b e or t o reduce the absorbance t o a linear region by dilution of t h e sample or by use of a smaller path length absorption cell. T h e relative precision is 3% for 0.05-10 p p b Hg(II), a n d it is better t h a n 10% down t o 5 p p t with the 60-cm long cell. T h e precision of analysis is limited by shot and source flicker noise below 0.05 p p b Hg(II), and above 0.05 p p b Hg(I1) it is limited by the reproducibility of sample injection. T h e detection limits, defined as the concentration which yields an absorbance twice t h a t of the standard deviation of the absorbance of a blank, are 3 p p t a n d l ppt, for t h e 20-cm and 60-cm long cells, respectively.
ANALYSES Atomic absorption determination of the mercury content in t a p water, in house distilled water, in double distilled water, in 1% (v/v) HCl (concd), and in Willamette River water were made with the 60-cm absorption cell. T h e water samples were analyzed directly and also after addition of acid a n d permanganate. In the latter case, to 490 ml of the (concd) a n d 5 ml of water sample were added 5 ml "03 0.2% (w/v) K M n 0 4 a n d the samples were analyzed immediately. N o difference in peak height was noted if the samples were analyzed 15 hours after treatment. T h e d a t a shown in Table V indicate t h a t the apparatus can be used for the direct analysis of water samples after treatment with the acid-permanganate which releases the mercury, t h a t each distillation step reduces the mercury content by about a factor of 3, a n d t h a t our HCl (concd) has about 1 ppb Hg(I1) which is considerably more t h a n in HNO3 (concd).
DISCUSSION With a few basic modifications t o t h e normal apparatus for the cold vapor atomic absorption determination of mercury, t h e detection limit, the sample volume, and the analysis time have been reduced significantly. T h e improvement in the technique is due largely t o t h e unique design of the reduction vessel which allows t h e mercury t o be swept efficiently out of the solution into the small volume of carrier gas to provide a much more concentrated mercury plug compared t o previous reduction vessels. T h e new reduction vessel, coupled with the small sample volumes, small apparatus dead volume, and optimized absorption cell dimensions, results in a greater calibration sensitivity or larger peak absorbance per p p b or per ng Hg(I1). For a given length absorption cell, it is shown t h a t the total cell volume must be reduced to some critical volume t o maximize the absorbance. T h e increased calibration sensitivity and the low noise a n d drift in the double beam AA system result in extremely low detection limits as evidenced by comparison t o detection limits for the normal apparatus. For AA, the concentration detection limit has been reduced from 0.02 p p b ( 4 ) t o 1 p p t Hg(II), and the absolute detection limit has been reduced from 0.2 ng ( I O ) t o 1 pg of mercury. These low detection limits make it possible for the first time to determine mercury levels on the order of 10 p p t which occur in natural waters, by the cold vapor AA technique without preconcentration methods. T h e capability to carry out direct analysis saves considerable time and prevents contamination or loss of mercury t h a t can occur in concentration procedures. With the mercury hollow cathode t u b e normally used, the detection limit would be a t least an order of magnitude worse because of shot noise limitations. T h e use of the much more intense mercury pen lamp operated in the unconventional dc current mode and of double beam compensation allows peak absorbances of lo-* absorbance unit t o be detected.
T h e fast analysis time of 2 minutes per sample is achieved through t h e reduction vessel design which utilizes small sample volumes (100 times less t h a n some systems) t h a t can be handled quickly and which allows rapid evacuation of t h e sample after analysis; since the reduction vessel does not have t o be replaced between runs as in some systems, automation should be easy t o implement. T h e problems of mercury loss and contamination a t the sub-ppb levels studied are critical. A 1% (v/v) HNO3 (concd) 0.002% (w/v) K M n 0 4 solution was shown t o be adequate for storing standard Hg(I1) solutions for a day without contamination. All glassware with which sample solutions come into contact must be rinsed with oxidizing solutions t o remove mercury adsorbed on the surface. Memory effects from glassware in contact with more concentrated Hg(I1) solutions can be limiting unless scrupulous rinsing is performed. T h e system should be useful for monitoring mercury in natural unpolluted waters a n d for studies of interesting chemical and physical phenomena a t s u b - p p b levels. T h e mechanisms of mercury loss and contamination are still uncertain. T h e effects of preservation techniques and oxidizing solutions on different organomercury compounds or on bound mercury are unclear. Instrumental parameters have been optimized t o provide the lowest detection limit, although the actual value depends upon the final choice of cell path length a n d sample size. If necessary, it may be possible t o reduce t h e detection limit by a factor of two or three by further minimization of dead volume in the reaction cell a n d tubing, by modification t o a closed system, or by reduction of lamp flicker noise. T h e improvements made in this technique are not restricted t o mercury determinations b u t are applicable t o other systems which are based on the reduction and/or diffusion of an analyte from a solution into a carrier gas stream before detection. T h e reduction vessel can also be used with atomic fluorescence or plasma emission systems, and the AF system has recently been evaluated for Hg determinations (15).
+
ACKNOWLEDGMENT T h e authors thank S. E. Ingle for many helpful comments related t o this manuscript.
LITERATURE CITED D. C. Manning, At. Absorpt. News/., 9, 97 (1970). R . Rehfus and A. Priddy, "Mercury Contamination in the Natural Environment," U S . Department of the Interior, Office of Library Services, July 1970. "Mercury in the Natural Environment: A Review of Recent Work," Geological Survey of Canada, M44-70-57. 1970, 39 pp. J. F. Kopp, M. C. Longbottom, and L. B. Lobring, J. Am. Wafer Works Assoc., 64, 20 (1972). H. W. Harvey, "Chemistry and Fertility of Sea Waters," 26 ed.. London, Cambridge University Press, 1963, 234 pp. J. Olafasson, Anal. Cbim. Acta., 68, 207 (1974). C. E. Knapp, Envir. Sci. Tech., 4, 890 (1970). R . A. Carr, J. B. Hoover, and P. E. Wilkniss, Deep-sea Res., 19, 747 (1972). V. Chau and H. Saitoh, Environ. Sci. Techno/., 4, 839 (1970). J. H. Hwang, P. A. Ullucci, and A. L. Malenfant, Can. Specfrosc., 16, 2 (1971). T. R. Gilbertand D. N. Hume, Anal. Cbim. Acta, 65, 461 (1973). M. P. Stainton. Anal. Cbem., 43, 625 (1971). J. F. Uthe. F. A. J. Armstrong, and M. P. Stainton. J. Fish Res. Board Can., 27, 805 (1970). C. Feldman, Anal. Cbem., 46, 99 (1974). J. E. Hawley and J. D. Ingle, Jr., Anal. Cbim. Acta., in press.
RECEIVEDfor review July 12,1974. Accepted December 12, 1974. Presented in part a t the 1974 Northwest ACS meeting. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of this research. ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
723
Modifications to the normal apparatus used for the cold vapor atomic absorption determination of mercury have been made by reducing the dead volume of the apparatus, by increasing the efficiency of diffusion of elemental mercury into the carrier gas, and by optimizing the instrumental parameters. The time of analysis, the sample volume, and the detection limit have been greatly reduced. For a 1-ml sample, the concentration detection limit is 1 ppt of Hg(ll). The relative standard deviation for 0.05-10 ppb of Hg(ll) is approximately 3 %. At concentrations below 0.05 ppb Hg(ii), imprecision is due mainly to the fluctuations from the radiation source; and, at concentrations above 0.05 ppb Hg( ll), sampling imprecision is limiting. To prevent mercury losses, an oxidizing agent and acid should be used as a preservative and, to prevent contamination, ail glassware must be adequately cleaned with the oxidizing solution.
Mercury in the environment is of great current interest because of its toxicological effect on plant and animal life. Sources of mercury are man-made, such as industrial wastes, pesticides, and fungicides, or natural, such as mineral deposits. T o pinpoint the sources of mercury contamination and to evaluate the levels of mercury contamination in a host of materials, extensive research on mercury determination has been conducted as evidenced by a number of review articles (1-3). In particular, the cold vapor atomic absorption method has received the greatest attention because this technique normally provides detection limits on the order of 1.0 p p b and is relatively simple and free from most interferences. T h e best detection limit reported ( 4 ) for this technique is about 0.02 p p b Hg(I1). Determination of residual mercury in natural water is a particular problem because mercury levels in natural water are often 0.03-0.06 p p b or lower and tedious preconcentration procedures are necessary (5-9). These facts indicate the need for a more sensitive instrument for the direct determination of mercury in natural waters. A critical study of the cold vapor method for atomic absorption determination of mercury was made in order to improve the instrumentation, t o lower the detection limit for mercury, and to improve the precision of analysis a t sub-ppb mercury concentrations. In addition, a more fundamental understanding of variables t h a t affect the technique and of the factors t h a t affect the precision and accuracy of analysis was desired.
EXPERIMENTAL Solution and Glassware Preparation. The stock solutions were prepared from reagent grade chemicals and distilled water as indicated in Table I. The SnC12 solution was bubbled with nitrogen to remove mercury contamination. All standards contained 1% (v/v) "03 (concd) and 0.002% ( w i v) KMn04 as a preservative and were prepared by dilution of the 100-ppm mercury stock solution. They were analyzed within eight hours of preparation.
To whom correspondence should be addressed
Glassware was soaked in a nitric acid-permanganate solution for 24 hours. Afterwards, the glassware was washed with HC1 (concd),
(concd), and rinsed rinsed with distilled water, washed with "03 with distilled water. Instrumentation. The final instrumentation selected is shown in Figure 1 and the specific components are identified in Table 11. The lower half of Figure 1 shows the basic elements of a doublebeam AA instrument with direct linear absorbance readout. Initially, a single-beam system was used. However, the improvement in long-term stability and the reduction of short-term source flicker noise made the double-beam system preferable for convenience and for the best precision and detection limits. The dc lamp power supply for the pen lamp was constructed from a 0-600 voltage source in series with a 20-K ballast resistor and a 20-mA current meter. The lamp was enclosed in a small cardboard box to reduce the effect of air currents on the lamp temperature and, hence, on the lamp radiance. For dc operation, the lamp is warmed up for 30 minutes with the ac transformer and started with a vacuum tester. The polarity of the electrodes should be changed every two weeks to ensure a long lifetime and stability. The voltage amplifier was constructed from an OA, a power supply, and appropriate connectors mounted in a metal box. Rotary switches allow selection of various feedback precision resistors, of feedback capacitors, of different input precision resistors, and, hence, of the voltage gain and RC time constant. The logarithmic ratio amplifier was constructed from a commercial log ratio module, power supply, and connectors mounted in a metal box, and it produces 1 volt per absorbance unit. The beam splitter is a quartz window placed in the center of a sample holder. A small percentage of the source radiation is reflected 90' and passes through a 3mm aperture and a 254-nm interference filter to the reference photomultiplier tube. The upper half of Figure 1 shows the flow system. During analysis, the carrier gas, air, flows from the compressed air tank (breathing quality) and regulators through a T-bore stopcock, the reduction vessel, a drying tube, the absorption cell, another T-bore stopcock, a flowmeter, two mercury oxidizing traps (chromic acid and acid-permanganate solutions in filtering flasks), and finally into the hood. The two T-bore stopcocks can be turned 90' to allow the solution in the reduction vessel to be evacuated into the filter flask without drawing the oxidizing solutions in the traps back into the line. The drying tube is made of Tygon tubing, filled with Mg(C104)2, and sealed with quick-connect ends. The reduction vessel was constructed from a glass sealing tube with a 1-cm porous frit. One end is tapered for connection to 3/le;-in. i.d. tubing and the other end is widened to 15-mm 0.d. to house tightly a No. 20 sleeve type rubber stopper for injection of samples. There is a glass exit tube 2 cm beneath the stopper and 8 cm above the frit. Pointed impressions, about 4 cm from the top, stop bubbles from creeping out the exit tube. The absorption cell was constructed by attaching 1-inch diameter quartz windows with apiezon wax to the ends of glass tubing. Inlet and outlet ports (3/16-in.0.d.) were attached about 1 mm from each end. Different lengths and cell diameters were studied as discussed in a later section. For alignment of the absorption cell, it is mounted with tube holders on vertical translators upon an optical rail secured to a horizontal translation stage. Procedure. The final procedure used for analysis and for optimization is discussed below. Instrumental variables are adjusted to the values noted in Table I11 after optimization of the cell position. Both the sample and reference photomultiplier power supplies are adjusted to produce 10-5-A photoanodic currents. The voltage amplifier gain is adjusted so as t o give a deflection at least half of full scale (1 V) on the recorder. One-tenth of a milliliter of reductant is injected with a 1-ml syringe into the reduction vessel after the carrier gas flow has been initiated through the frit. The bubbling of the reductant removes mercury that may be present. After the reANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
*
719
Table I. Solution Preparation
Table 111. Optimal Values for Analysis
Reductant: 1 g SnC1, -t 1 m l HCl(concd) diluted t o 100 in1 [I% (w/v) S ~ C I , ] Oxidant: 0.2 g KMn04 diluted t o 100 m l [0.2% (w/v) m n o ,I Mercury: 0.1354 g HgC1, + 50 m l HNO,(concd) diluted to 1 1. [lo0 ppm Hg(I1) stock solution]
Table 11. Components of Instrumentation Item
1. Sources and power supplies M e r c u r y pen l a m p
Supplier and Model
Ultra- Violet Prod.
- 11sc-1c
2. 3.
4.
5.
6. 7.
a.
AC power supply DC power supply Mercury hollow cathode tube Monochromator Photomultiplier tubes, p o w e r supplies, and holders Sample PMT Power supply Holder Reference PMT Power supply Holder Log r a t i o amplifier Log r a t i o module Power supply Voltage amplifier Operational a m plifier Power supply Strip chart recorder F 1o w m e t e r Flow r e g u l a t o r s
Beam s p l i t t e r Housing Interference filter (254 nm) Quartz window 10. Sleeve type r u b b e r stopper
Ultra-Violet Prod. -SCT- 1 E/M CO. Westinghouse-WL 22847A Heath- EU- 700/E
Value
Gas carrier Volume of reductant Volume of sample Slit width Absorption cell Radiation s o u r c e Lamp current RC t i m e constant Voltage amplifier gaina Photoanodic current Photomultiplier supply voltagea a
140 ml/min Medium 5-cm long x 12-mm diameter Mg(C104), Air 0.1 m l 1.0 m l 1000 p m 20-cm long x 2-mm i.d. 60-cm long x 2-mm i.d. Hg pen lamp (dc) 9-10 mA 0.32 sec 1-100 10-5 A 500-600 V
Adjusted as described in procedure.
RESSED
I
RCA- 1P2 8 Heath- EU-42A Heath- EU- 701- 93 HTV-R166 s o l a r blind Keithley- 84489 McKee-Pedersen-MP-1021
Teledyne- Philbrick-4361 Analog Devices-915 SUPPLY
RECORDER
SUPPLY
Function Modules-3801 Analog Devices-915 Heath-EU-205-11 Gilmont- F 7260 Matheson-70 Victor-SR- 200
1
t
1
Figure 1. Instrumentation
9.
McKee- Pedersen-MP- 1017 Pomfret R e s . Optics-20-2357- 1 ESCO Optics Prod. VWR-16170-167 ~
corder pen has returned to the base line, 1 ml of Hg(I1) standard is injected with a 1-ml syringe into the reduction vessel. The absorption peak is similar to that previously reported ( I O ) , except the peak width is much smaller. The beginning of the peak is observed about 3 seconds after injection. Within 6 seconds, the peak absorbance IS recorded and, within 20 seconds, the pen has returned to the base line. For measurement of the peak absorbance, a peak base line is interpolated by drawing a line between the base lines before injection and after the peak has passed. The difference between the absorbance at the peak and the absorbance of the interpolated base line is taken as the peak absorbance. The sample then is evacuated and a 1-ml blank is flushed through the frit to eliminate memory effects above 1 ppb Hg(I1) levels. The first stopcock is rapidly alternated between vacuum and carrier gas a few times to remove most of the solution from the frit. The rubber stoppers and the drying tube are replaced after 20 injections and samples can be analyzed at a rate of about 30 per hour. Optimization. General. In this section, the studies of the effects of various instrumental or physical variables on the observed peak height, width, and shape, on reproducibility, and on S/N are discussed. The effects of the variables will be discussed one at a time. 720
Variable
Flow rate Frit g r a d e Drying tube
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4 , APRIL 1975
Unless stated otherwise, a 10-ppb Hg(I1) standard, a 20-cm long, 5-mm diameter absorption cell, and the other settings for the variables listed in Table I11 were employed. The basic analysis scheme described in the previous section was used for all optimization studies. To understand the influence of different variables on the data and to understand why the described system is superior to previous systems in a number of respects, one must first understand the basic processes that occur in analysis. When a Hg(I1) solution is injected into the reduction vessel with the bubbling SnC12 solution, the Hg(I1) ions are reduced to neutral mercury atoms which diffuse from the solution into the carrier gas and are carried through the reduction vessel to the drying tube and finally to the absorption cell. Essentially all of the mercury atoms diffuse from the solution into a given volume of carrier gas. This volume of carrier gas which contains mercury is denoted the plug. Ideally, the plug would be of uniform concentration but in reality the plug-concentration profile appears to have a maximum with respect to time. The diffusion out of the reduction vessel will be controlled by the size and shape of the reduction vessel, the volume of reductant and Hg(I1) solution, the efficiency of aeration, the effective solution surface area during aeration, and the rate constant for diffusion of mercury through a solution of unit depth into the carrier gas. If diffusion of mercury in the carrier gas is important, then the volume of the plug is expected to increase as it moves through the apparatus. The plug of mercury will first be observed when it enters the front of the cell and the maximum value will be observed after the plug reaches the end of the cell and the most concentrated part of the plug is in the cell. The observed peak height or peak absorbance will be dependent on the effective concentration of the plug or of the portion of the plug in the cell. The observed width will depend on the volume of the mercury plug entering the cell and the cell volume. If either volume is reduced, the observed width
I
I
< 120
4
0 90L 24
9 m
0221
0 80
-
~
b 070-
060-
'
I 400
SA0
700
660
Flow r o t e (ml/minI
Figure 2. Relationship of peak parameters to flow rate (e) Half-width(sec), (3 Half-width (mi), and (91 Peak absorbance Internal d i o m e l e r
mml
Figure 4. Effect of cell dimensions on peak absorbance (kj 20-cm length, (Q 60-cm length, and (m) 120-cm length 130-
09CF
i 080L
o
t
z
t
0 200
1 !
.,+ cc
, 04
(12
06
08
I
. ,o
VOLUME 0 1 STbNDARO
[rnl
101
10nqi
Figure 3. Effect of solution volume in reduction vessel on peak absorbance (h) 1.0 mi of SnCi2, (4 0.5 mi of SnCi2, and (j0.1 ml of SnCi2
will be reduced. However, if the cell volume is much smaller than the plug volume, the cell volume should have little effect on the observed width. If the flow rate of carrier gas is doubled but the volume of the plug is unchanged, then the mercury will diffuse out of the solution twice as fast, the velocity of the plug through the system will be twice as fast, and the peak width in time will be reduced. Other authors ( I 1-13) have indicated the importance of maximizing the efficiency of diffusion of mercury into the carrier gas, minimizing the dead volume, and getting the plug completely into the cell to achieve the most concentrated mercury plug. Most researchers ( 4 , 6, 8, 10, .12, 13) have used rather large reduction vessels with inefficient aeration. Recently, Gilbert and Hume (11) reported a reduction vessel similar to the one used in this research. The carrier gas was hubbled through the frit of a Buchner funnel into the solution. The detection limits were much greater than those reported here because of the much larger size of their system. F l o u Rate. The effect of carrier gas flow rate on the peak absorbance, the peak half-width in seconds, and the peak half-width in milliliters (i.e., the product of the half-width in seconds times the flow rate) is shown in Figure 2. The results are similar to those previously reported (10, 11, 13)and also indicate that about 90% of the mercury is in a 20-ml plug of carrier gas and that a flow rate of about 140 mllmin is optimal. Volume Study. The effect of the volume of solution in the reduction vessel on the peak absorbance for a constant absolute mass (10 ng) of mercury was studied. This was done by mixing 0.1 ml of 100 ppb, 0.5 ml of 20 ppb, or 1.0 ml of 10 ppb of a Hg(I1) standard solut,ion with 0.1 ml, 0.5 ml, or 1.0 ml of the SnClz solution. The results shown in Figure 3 are similar to those previously reported
L A -__ . _ ~ 1 . 0 I0 20 30 40 50 C o n c e n t r O l l O n (pph]
Figure 5. Effect of the type of lamp and lamp conditions on calibration curves Pen lamp: (n) 14 mA ac, ( 0 ) 10 mA dc, and ( p ) 6.5 mA dc. Hollow cathode: (4) 10 mA dc and (6 5 mA dc
( I O , 11) and clearly illustrate that the peak absorbance increases with decreasing volume in the reduction vessel because of faster diffusion rate of mercury atoms from the solution into the air stream. One milliliter of standard or sample and 0.1 ml of reductant were chosen for final solution parameters. Smaller volumes of Hg(I1) standards increase the absolute response (Alng) but produce a much poorer concentration response (Alppb) and are more dependent on the reproducibility of sample volumes added. Absorption Cell. Alignment of the radiation source with respect to the slit of the monochromator is made by positioning the lamp in both vertical and horizontal directions to achieve the greatest photoanodic current. The absorption cell is then placed in the tube holders upon the optical rail and is oriented in the X-Y plane with the translators to achieve a maximum photoanotlic current. The peak absorbance was measured for absorption cells which varied in length from 20 to 120 cm and in internal diameters from 18.6 mm to 1 mm, and the results are shown in Figure 4. For a given cell length, the absorbance increases with decreasing cell diameter and approaches a limiting value. The increase in absorbance, as the cell diameter decreases, occurs because the average concentration in the absorption cell increases as the volume of the absorption cell decreases until the cell volume becomes small enough to contain the most concentrated part of the mercury plug. The volume of the ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4 , APRIL 1975
721
Table I V . Percent Losses of M e r c u r y over 24 H o u r s Percent loss O.~%(V/V)
1% ( v i v ) Hh'O3 (concd),
H20, polyethylene Solution
Blank 0.01 ppb 1.0 ppb 10 PPb
polyethylene
-
0.002% ( w / v )
KhtnO4, borosilicate
(concd)
"03
+ 0.0006% (w/v)
KMnO4,
borosilicate
Closed
Open
Closed
Open
closed
closed
... ...
...
...
...
80 28
90 28
60 20
80 20
16 6 10 1
29 29 28 12
...
CHEMICAL A N D PHYSICAL PROBLEMS I%(II) PPt
Sample
T a p water Distilled Twice distilled 1% (v/v) HC1 (concd) Willamette R i v e r
...
...
Table V. W a t e r A n a l y s i s Without preservative
0 0 0 15 0
With preservative
10 3 -1
... 9
plug was estimated (twice the half-width in ml) t o be about 20 ml. The total volume of the cell must be less than about 4 ml to achieve near maximum peak absorbance. For a constant cell diameter of 2 mm, the signal increased 2.6 times from the 20-cm length to the 60-cm length and 3.5 times from the 20-cm length to the 120-cm length. The increase in absorbance with cell length for small diameters can be explained in terms of Beer's law. The absorbance is directly proportional to the length if the analyte concentration is uniform throughout the cell. The negative deviations from Beer's law evident for the 60-cm and 120-cm long cells may be due to the non-uniform mercury plug concentration profile from the 10-ppb Hg(I1) solution. However, at mercury concentrations below 1 ppb, the absorbance with the 60cm length cell is 3 times the absorbance with the 20-cm length cell, thus conforming to Beer's law. For practical purposes, the 120-cm long and 1-mm diameter absorption cells are very difficult to align and it is difficult to maintain the alignment. Therefore, 2-mm diameter absorption cells with 20-cm and 60-cm lengths were chosen as the optimum dimensions. Radiation Sources. A mercury hollow cathode tube and a mercury pen lamp were evaluated as atomic absorption sources with the 2-mm X 20-cm length cell. Figure 5 shows the calibration curves constructed from peak absorbance measurements for three different Hg(I1) concentrations for the two lamps operated under different current conditions. The data indicate that the slope or sensitivity in absorbance units per ppb and the linearity are better for the hollow cathode tube, for lower currents with either lamp, and for dc rather than ac operation with the pen lamp. Below 6.5 mA dc, the pen lamp will not operate. Although the hollow cathode tube provides the greatest slope, a lens was needed to focus and collimate the radiation to produce about one tenth the light level impinge upon the photomultiplier tube as produced with the pen lamp without optics, and the signal from the hollow cathode radiation was shot noise limited. For further analysis, the mercury pen lamp was chosen since it was more intense, since it could be used with longer 2-mm diameter absorption cells, and since it was easier to align. Finally, a 9-10 mA dc current was found to provide the best stability. The negative deviations observed in Figure 5 may be ascribed (13) to differences in the emission line width of the lamps operated under different current conditions and hence at different temperatures. Reduction Vessel. Thr'ee grades of porous frits for the reduction vessel, coarse, medium, and fine, were studied. The peak absorbance obtained with the fine frit was only 2% higher than for the medium frit and 5% higher than for the coarse frit. A medium frit was chosen for all succeeding studies since the fine frit easily clogs and requires the most time to evacuate. A reduction vessel made from a 30-mm diameter medium frit sealing tube with reduced ends gave about a 13% lower peak height than the 10-mm diameter coarse fritted reduction vessel. 722
1% (v/v) HNO3 (concd)
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
S t o r a g e Losses. Mercury losses with polyethylene a n d borosilicate glass containers were studied under different conditions. Feldman (14) presents a n excellent study of t h e preservation of dilute mercury solutions down t o 0.1 p p b a n d reviews previous work in this area. Until this study, all work had been done a t mercury concentrations above 20 ppb. Feldman's a n d our d a t a show that t h e percent mercur y loss with respect to time a n d t h e effectiveness of preservatives are very dependent o n t h e initial mercury concentration. T h e d a t a shown in Table IV for percent loss over a period of 24 hours from time of preparation indicate t h a t t h e 1%(v/v) "03 (concd) 0.002% (w/v) K M n 0 4 preservative is t h e most suitable for mercury storage. For this period of time and especially for longer periods of storage, t h e HN03-KzCr20, preservative suggested by Feldman ( 1 4 ) appears to be superior. M e r c u r y C o n t a m i n a t i o n a n d Blank P r o b l e m s . T h e blank solution will give a positive absorbance reading or mercury solutions will give erroneously high absorbances if t h e blank or mercury solutions pick u p mercury. Mercury contamination results from residue mercury in t h e distilled water, in t h e reagents, from t h e air, or from t h e surfaces of t h e volumetrics, of t h e reduction vessel, or of t h e syringes. T h e cleaning procedures outlined in t h e procedure section should reduce t h e extent of contamination from some of t h e sources. Untreated house-distilled water produces n o blank signal. T h e following solutions were made from house-distilled water: a 1%(v/v) "03 (concd) solution, a 0.002% (w/v) K M n 0 4 solution, and a 1% (v/v) "03 (concd) 0.002% (w/v) K M n 0 4 solution. T h e absorbances of t h e above three solutions were measured. Only t h e last solution gave a measurable blank value and, after diluting this solution 1:2 with house-distilled water, t h e blank value remained constant. Therefore, it was concluded that in t h e strong oxidizing environment of t h e acid-permanganate preservative solution, t h e mercury was released from organomercury compounds, bacteria, or particulate matter in t h e distilled water. Finally, double distilled water in t h e preservative solution exhibited t h e same characteristics as stated above; however, t h e blank value was reduced by about one third.
+
+
RESULTS For t h e 20-cm cell, t h e slope of t h e AA calibration curve is 0.0219 ppb-1 and is linear u p t o about 10 p p b of Hg(I1). At 50 p p b Hg(II), t h e slope of t h e calibration curve is about a factor of three less. For t h e 60-cm cell, t h e slope of t h e calibration curve is 0.0638 ppb-' a n d is linear u p to aboilt 3 p p b Hg(I1). As illustrated by Figure 5 , serious negative deviations occur for concentrations which yield absorbances above 0.5. T o analyze solutions of higher Hg(I1) concentrations, i t would be desirable either t o use t h e pen lamp a t 6.5
mA dc or the hollow cathode t u b e or t o reduce the absorbance t o a linear region by dilution of t h e sample or by use of a smaller path length absorption cell. T h e relative precision is 3% for 0.05-10 p p b Hg(II), a n d it is better t h a n 10% down t o 5 p p t with the 60-cm long cell. T h e precision of analysis is limited by shot and source flicker noise below 0.05 p p b Hg(II), and above 0.05 p p b Hg(I1) it is limited by the reproducibility of sample injection. T h e detection limits, defined as the concentration which yields an absorbance twice t h a t of the standard deviation of the absorbance of a blank, are 3 p p t a n d l ppt, for t h e 20-cm and 60-cm long cells, respectively.
ANALYSES Atomic absorption determination of the mercury content in t a p water, in house distilled water, in double distilled water, in 1% (v/v) HCl (concd), and in Willamette River water were made with the 60-cm absorption cell. T h e water samples were analyzed directly and also after addition of acid a n d permanganate. In the latter case, to 490 ml of the (concd) a n d 5 ml of water sample were added 5 ml "03 0.2% (w/v) K M n 0 4 a n d the samples were analyzed immediately. N o difference in peak height was noted if the samples were analyzed 15 hours after treatment. T h e d a t a shown in Table V indicate t h a t the apparatus can be used for the direct analysis of water samples after treatment with the acid-permanganate which releases the mercury, t h a t each distillation step reduces the mercury content by about a factor of 3, a n d t h a t our HCl (concd) has about 1 ppb Hg(I1) which is considerably more t h a n in HNO3 (concd).
DISCUSSION With a few basic modifications t o t h e normal apparatus for the cold vapor atomic absorption determination of mercury, t h e detection limit, the sample volume, and the analysis time have been reduced significantly. T h e improvement in the technique is due largely t o t h e unique design of the reduction vessel which allows t h e mercury t o be swept efficiently out of the solution into the small volume of carrier gas to provide a much more concentrated mercury plug compared t o previous reduction vessels. T h e new reduction vessel, coupled with the small sample volumes, small apparatus dead volume, and optimized absorption cell dimensions, results in a greater calibration sensitivity or larger peak absorbance per p p b or per ng Hg(I1). For a given length absorption cell, it is shown t h a t the total cell volume must be reduced to some critical volume t o maximize the absorbance. T h e increased calibration sensitivity and the low noise a n d drift in the double beam AA system result in extremely low detection limits as evidenced by comparison t o detection limits for the normal apparatus. For AA, the concentration detection limit has been reduced from 0.02 p p b ( 4 ) t o 1 p p t Hg(II), and the absolute detection limit has been reduced from 0.2 ng ( I O ) t o 1 pg of mercury. These low detection limits make it possible for the first time to determine mercury levels on the order of 10 p p t which occur in natural waters, by the cold vapor AA technique without preconcentration methods. T h e capability to carry out direct analysis saves considerable time and prevents contamination or loss of mercury t h a t can occur in concentration procedures. With the mercury hollow cathode t u b e normally used, the detection limit would be a t least an order of magnitude worse because of shot noise limitations. T h e use of the much more intense mercury pen lamp operated in the unconventional dc current mode and of double beam compensation allows peak absorbances of lo-* absorbance unit t o be detected.
T h e fast analysis time of 2 minutes per sample is achieved through t h e reduction vessel design which utilizes small sample volumes (100 times less t h a n some systems) t h a t can be handled quickly and which allows rapid evacuation of t h e sample after analysis; since the reduction vessel does not have t o be replaced between runs as in some systems, automation should be easy t o implement. T h e problems of mercury loss and contamination a t the sub-ppb levels studied are critical. A 1% (v/v) HNO3 (concd) 0.002% (w/v) K M n 0 4 solution was shown t o be adequate for storing standard Hg(I1) solutions for a day without contamination. All glassware with which sample solutions come into contact must be rinsed with oxidizing solutions t o remove mercury adsorbed on the surface. Memory effects from glassware in contact with more concentrated Hg(I1) solutions can be limiting unless scrupulous rinsing is performed. T h e system should be useful for monitoring mercury in natural unpolluted waters a n d for studies of interesting chemical and physical phenomena a t s u b - p p b levels. T h e mechanisms of mercury loss and contamination are still uncertain. T h e effects of preservation techniques and oxidizing solutions on different organomercury compounds or on bound mercury are unclear. Instrumental parameters have been optimized t o provide the lowest detection limit, although the actual value depends upon the final choice of cell path length a n d sample size. If necessary, it may be possible t o reduce t h e detection limit by a factor of two or three by further minimization of dead volume in the reaction cell a n d tubing, by modification t o a closed system, or by reduction of lamp flicker noise. T h e improvements made in this technique are not restricted t o mercury determinations b u t are applicable t o other systems which are based on the reduction and/or diffusion of an analyte from a solution into a carrier gas stream before detection. T h e reduction vessel can also be used with atomic fluorescence or plasma emission systems, and the AF system has recently been evaluated for Hg determinations (15).
+
ACKNOWLEDGMENT T h e authors thank S. E. Ingle for many helpful comments related t o this manuscript.
LITERATURE CITED D. C. Manning, At. Absorpt. News/., 9, 97 (1970). R . Rehfus and A. Priddy, "Mercury Contamination in the Natural Environment," U S . Department of the Interior, Office of Library Services, July 1970. "Mercury in the Natural Environment: A Review of Recent Work," Geological Survey of Canada, M44-70-57. 1970, 39 pp. J. F. Kopp, M. C. Longbottom, and L. B. Lobring, J. Am. Wafer Works Assoc., 64, 20 (1972). H. W. Harvey, "Chemistry and Fertility of Sea Waters," 26 ed.. London, Cambridge University Press, 1963, 234 pp. J. Olafasson, Anal. Cbim. Acta., 68, 207 (1974). C. E. Knapp, Envir. Sci. Tech., 4, 890 (1970). R . A. Carr, J. B. Hoover, and P. E. Wilkniss, Deep-sea Res., 19, 747 (1972). V. Chau and H. Saitoh, Environ. Sci. Techno/., 4, 839 (1970). J. H. Hwang, P. A. Ullucci, and A. L. Malenfant, Can. Specfrosc., 16, 2 (1971). T. R. Gilbertand D. N. Hume, Anal. Cbim. Acta, 65, 461 (1973). M. P. Stainton. Anal. Cbem., 43, 625 (1971). J. F. Uthe. F. A. J. Armstrong, and M. P. Stainton. J. Fish Res. Board Can., 27, 805 (1970). C. Feldman, Anal. Cbem., 46, 99 (1974). J. E. Hawley and J. D. Ingle, Jr., Anal. Cbim. Acta., in press.
RECEIVEDfor review July 12,1974. Accepted December 12, 1974. Presented in part a t the 1974 Northwest ACS meeting. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of this research. ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
723
