Automated Method for Determination of Mercury B. W. Bailey and F. C. Lo DiGision of Laboratories and Research, New York State Department of Health, Albany, N . Y . 12201 THEGENERALLY accepted procedure for determination of trace amounts of mercury is the cold vapor atomic absorption technique originally described by Hatch and Ott ( I ) . This method, however, is relatively time-consuming and the receipt of a considerable number of samples for mercury determination at the authors' laboratory (prompted by the current concern with mercury pollution) led to the development of an automated version of the Hatch and Ott method which is described. EXPERIMENTAL Equipment. The equipment used in this work was a Varian Techtron atomic absorption spectrophotometer model AA5 with auto sampler, curve corrector, digital indicator, and digital printer, together with a Technicon Mark I1 peristaltic action pump and associated glassware and tubing. Reagents. STANDARD MERCURY SOLUTION.A 1000-ppm mercury stock solution is prepared by dissolving 1.3538 grams of mercuric chloride in a small amount of water, adding 7 ml of concentrated sulfuric acid to avoid hydrolysis of the mercury salt, and finally diluting the mixture to 1 liter with deionized water. By proper dilution, a fresh 10-ppm stock solution is prepared daily from which a 1-ppm solution is prepared twice a day to provide mercury standards. STANNOUS CHLORIDE SOLUTION.A fresh solution is prepared daily by dissolving 100 grams of stannous chloride in a small amount of water, adding 14 ml of concentrated sulfuric acid, and then diluting with deionized water to make a 10% solution. HYDROXYLAMINE HYDROCHLORIDE-SODIUM CHLORIDE SOLUTION. Sixty ml of 2 5 % hydroxylamine hydrochloride are mixed with 50 ml of 30z sodium chloride and diluted to 500 ml with deionized water. POTASSIUM PERMANGANATE SOLUTION.Fifty grams of potassium permanganate are dissolved in deionized water to make a 5 % solution. Procedure. Basically, the procedure followed is the same as the manual version in that the sample is digested to destroy organic matter and get the mercury into solution in the mercuric state. To this solution permanganate is added, followed by hydroxylamine hydrochloride and stannous chloride. This latter step reduces the mercury to the metal which can then be liberated as a vapor by aerating the solution with air or nitrogen. The absorbance of the mercury vapor so liberated is then measured. RESULTS AND DISCUSSION
The system used is shown schematically in Figure 1. The major difference from the Hatch and Ott manual procedure is that the closed system they employ for the aeration and subsequent liberation of the mercury vapor is not feasible in an automated method. In the manual method, the rate of aeration of solution is not particularly critical while in the automated system it is. For a given set of experimental conditions, the sensitivity decreased with increasing flowrate. However, with slower flow rates, it takes a longer time for the signal to return to the base line between samples. Thus, a compro-
mise must be made between sensitivity and speed of analysis. Under the experimental conditions described, a flow rate of 120 cc/min provided more than adequate sensitivity and it took 1 minute for the signal to return to the base line after a sample. The sampler, as supplied by the manufacturer, is intended for use in conventional atomic absorption spectrometry and has a very short sample time (25 sec/sample, maximum). This was modified by paralleling the capacitor in the timing circuit of the sampling cycle with a 5-pf capacitor which increases the sampling time to 1.1 minutes. Toward the end of the sampling cycle, the auto sampler triggers the printer. To ensure that the signal being produced is synchronized with the cycle, a simple adjustment is made in the lengths of tubing in the mixing stage of the procedure. In the system described, the signal attained a maximum about 2 minutes after sampling. Thus the output of the printer is always one sample behind the auto sampler. Since it takes a minute for the signal to return to the base line after a sample, a blank of deionized water is placed between each sample. The automatic control which is activated by the sampler zeroes the instrument between samples. This ensures compensation for base-line drift caused by lamp fluctuations or electronic variation between sample readings. The auto zero triggers after each sample cup on the auto sampler and will thus trigger twice between samples. After a sample has been read, the trigger will adjust the gain of the amplifier so that signal will correspond to zero; as the sample is purged, the signal on the amplifier will not be activated until mercury is completely dispelled from the system. At this point the auto zero will trigger again and the gain on the amplifier will be adjusted to give zero signal. This is best appreciated by monitoring the output on a recorder; an example is shown in Figure 2. The sample size required for'analysis is 7 ml of solution. The concentration range in which the authors were working was 0.001 to 0.01 ppm in which a scale expansion of about 7 X was used. It is thus possible to work in higher or lower concentration ranges by varying the scale expansion. The digital corrector allows adjustment to be made for any deviations from Beer's law. However, in the concentration range Table I. Printer Output of Automated Mercury Analyses Sample No." Reading, ppm Sample No a Reading, ppm ooO1 0003
0005 0007
00 00 00 01
o009
00
001 1 0013 0015 0017 0019 0021 0023 0025
00 00 01 00
00 00 01 00
28 57 88 17 23 59 86 16 23 54 91 20 26
0027 0029 0031 0033 0035 0037 0039 004 1 0043 0045 0047 0049
00 00 00 01 00 00 00 01 00
00 00 01
26 53 87 21 24 52 90 18 26 50 90 18
Even numbered samples are water blanks The corresponding readings are all 0 00.
(1) W. R. Hatch and W. L. Ott, ANAL.CHEM., 40,2085 (1968). ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
1525
a b
I ATOMIC DIGITAL INDICATOR
I.D. 0.065" 0.065"
-. .-..-. ..-..)METER
FLOW CELL II'LONG 3/8"ID
GAS LIQUID SEPARATOR IQUlD TO WASTE AUTOSAMPLER dlllh 7 C R n
I
- - I -
I
I
_-I..
I
PRINTER
Figure 1. Schematic of equipment used for automatic analysis of mercury a.
10% stannous chloride in 0.5N sulfuric acid
b. 3 % hydroxylamine hydrochloride-3 % sodium chloride c.
air
d. 5 % potassium permanganate e. sample
Table 11. Results of Duplicate Analyses by Automated Procedure and Comparisons with Manual Procedure Hg concentration, ppm Sample IO I Ib IIIC Brine 1 0.0077 0.0071
l n
t
f
t
$
+ve
E 2
0
2 3
m
a
9 U
-ve
Fish 1
+ ,
2
z
3 4
2 v)
I 1
2
3
TIME (MIN.)+
Figure 2. Output of automated system for analysis of mercury A and A' correspond to signal peaks 2 indicates triggering of auto zero B and B' are negative signals resulting from
0,0076 0.0044
0.07 0.4 0.8 1.2
0.06 0.4 0.6 1.0
0.055 0.062
Coald
a b
0.48 0.45 0.50 0.52
I. Automated procedure.
purging after gain has been zeroed on peaks
c
11. Beckman Mercury Vapor Monitor (Industrial Lab). 111. Beckman Mercury Vapor Monitor (Government Lab).
A and A'
d
Replicate analyses.
mentioned, it was unnecessary. The digital indicator unit enables the output of the amplifier to be displayed in terms of concentration. To determine the precision of the method, a series of standards was run in replicate. The standards contained 0.3,0.6, 0.9, and 1.2 pg mercury in 150 ml ( i x . , 2 to 8 ppb). The concentration readout was adjusted to display the output in terms of the amount of mercury present in the original solution. The printer output is shown in Table I. The coefficients of variation calculated from these results at the 2, 4, 6, and 8 ppb levels are 7.6,6.1,2.3,and 1 . 6 z , respectively. The procedure described combines rapidity, it is possible to analyze 22 samples per hour, and a high degree of precision and has the added advantage of requiring only a small sample for analysis. It has been satisfactorily applied to a wide variety of samples including water, coal, oil, blood, urine, hair, fish, and other foodstuffs. A comparison of results obtained with the procedure described, and independent laboratories, using manual proce-
1526
Bloodd
4
0,0078 0.0041
dures, is shown in Table 11, together with some results of replicate analyses of blood and coal samples. While the work described in this article was in progress, an automated procedure for determining mercury was presented at the Technicon Symposium (2). However, that procedure was developed for the continuous monitoring of water with a recorder output. ACKNOWLEDGMENT
The authors would like to acknowledge the technical assistance provided by Mr. E. Le Gere and Mr. T. Moran.
RECEIVED for review April 1, 1971. Accepted June 7,1971. (2) P. D. Goulden and B. K. Afghan, presented at the Technicon International Congress, New York, N. Y., November 1970.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
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