Automated Atomic Absorption Spectrometric Determination of Total Arsenic in Water and Streambed Materials Marvin Fishman" and Roberto Spencer
US. Geological Survey, Mail Stop 407, Box 25046,
Denver Federal Center, Denver, Colorado 80225
An automated method to determine both inorganic and organic forms of arsenlc in water, water-suspended mixtures, and streambed materials Is described. Organlc arsenic-containlng compounds are decomposed by either ultraviolet radiation or by sulfuric acid-potassium persulfate digestion. The arsenlc liberated, with inorganic arsenic originally present, Is reduced to arsine wlth sodium borohydride. The arsine is stripped from the solution with the aid of nitrogen and is then decomposed in a tube furnace heated to 800 O C which Is placed in the optical path of an atomic absorption spectrometer. Thirty samples per hour can be analyzed to levels of 1 pg arsenlc per liter.
Automated methods for the determination of inorganic arsenic at the microgram level in water by atomic absorption spectrometry using hydride generation have been reported. Goulden and Brooksbank ( I ) determined arsenic, antimony, and selenium using stannous chloride, potassium iodide, and aluminum for the hydride generation. The hydrides produced in the automated system are then passed to a tube furnace mounted in the light path of an atomic absorption spectrometer. The use of a tube furnace gives an increase in sensitivity over the conventional hydrogen-argon-entrained flame. The system will measure any inorganic arsenic present in the sample. T o determine arsenic in organo arsenic compounds, they first used a manual hydrochloric acidpersulfate pretreatment to decompose any organic compounds present. Pierce e t al. (2) employed sodium borohydride to convert selenium and arsenic to their respective hydrides. They used the stripping column suggested by Goulden and the tube furnace combustion concept. Again, only inorganic arsenic was determined. I t has been reported in the literature that some alkylated arsenicals can be determined without digestion. Braman and Foreback (3) separated and determined arsenic(III), arsenic(V), monoethylarsonic acid and dimethylarsonic acid based on sodium borohydride reduction of the arsenicals to their corresponding arsines. They used a specialized electrical discharge apparatus for the measurement of the arsines. Talmi and Bostick ( 4 ) used a similar technique followed by gas chromatography. More recently Edmonds and Francesconi ( 5 ) reported that the time consuming and possible error inducing digestion stage could be omitted from atomic absorption spectrometric procedures for estimating some alkylated arsenicals. When using the method of Pierce et al. (2),we were unable to obtain 100% recovery of some alkylated arsenicals (see Table I) and felt that a digestion step was necessary with their procedure. We have been unable to find any mention in the literature of an automated method that incorporates a procedure to first decompose organo arsenic compounds prior to the hydride formation. Automated methods for determining other complex organically bound substances have been reported. El-Awady, Miller, and Carter (6) described an automated method to determine organomercury in water and wastewater
using either acid persulfakdichromate or acid-permanganate digestions. An automated method to determine cyanide was reported by Goulden, Afghan, and Brooksbank (7). T o determine complex cyanides, they passed the solutions first through an ultraviolet digestor. The purpose of our study was to incorporate a digestion procedure into the automated arsenic procedure of Pierce et al. (2). Two automated digestion procedures are described: (1) ultraviolet radiation and (2) acid-persulfate digestion. The arsenic liberated by digestion, with inorganic arsenic originally present, is reduced to arsine with sodium borohydride. The arsine is stripped from the solution with the aid of nitrogen and is then converted to arsine ground state atoms in a tube furnace placed in the optical path of a n atomic absorption spectrometer. The stripping column and tube furnace used by Pierce were incorporated into this method. Thirty samples per hour can be analyzed. The method may be used to analyze waters and watersuspended sediment mixtures containing a t least 1 kg arsenic per liter. Samples containing more than 15 pg/L, must be diluted. The method may also be used to analyze bottom materials containing at least 1 pg arsenic per gram. For samples containing more than 15 pg/g, less sediment is used. Both inorganic and organic forms of arsenic are determined.
EXPERIMENTAL Apparatus. A P-E 603 atomic absorption spectrometer with arsenic electrodeless discharge lamp and P-E Model 056 stripchart recorder for data readout were used. The automated equipment consists of a Technicon sampler with stirrer, proportioning pump, ultraviolet digestor and/or heating bath. The ultraviolet digestor is used only for dissolved arsenic and cannot be used when analyzing water-suspended sediment mixtures or bed materials. The heating bath is used only in the acid-persulfate digestion procedure. The stripping column is identical to that used by Pierce et al. (2)and is packed with glass helices or beads. Because the stripping column is. not heated, it is not necessary to cool the condensing column. Nitrogen is introduced into the stripping column and is used to carry the arsine to the tube furnace. The flow rate is adjusted for maximum sensitivity by analyzing a series of identical standards. A flow rate of approximately 100 mL/min has been found satisfactory. The tube furnace is made from quartz tubing, 10 mm i.d. X 100 mm, with an eyelet at each end of the tube to anchor the nickel-chrome wire. The tube is fused at the center with a 2-mm i.d. quartz tube for the gas entry port. A ball and socket joint may be fused at the end of the condensing column and gas entry tube to facilitate in joining the two units together. The tube furnace is wrapped with 18 ft of 26-gauge nickel-chrome wire, covered with asbestos cloth and mounted in the optical path of the atomic absorption spectrometer. Reagents. The concentration of the reagents used in this method are as follows: hydrochloric acid (6 M), potassium iodide (100 g/L), potassium persulfate (50 g/L), sodium hydroxide (10 M), and sulfuric acid (3 M). The sodium borohydride solution is prepared by dissolving 5 g Na13H4 and 40 g NaOH in water and diluting to 1 L. Demineralized water is used to prepare these reagents. Procedure. Determine arsenic at 193.7 nm. Apply a voltage of 47 V or more (voltage regulator) as necessary to the tube furnace ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1599
Table I. Recovery of Arsenic from Organic Arsenic Compounds by Automated and Manual Procedures (All results in pg/L) Amount founda ~
Amount added
Compound Phenylarsonic acid Phenylarsonic acid Thorin (0-( 2-hydroxy-3,6-disulfo1-naphthy1azo)benzene arsonic acid) DSMA (Disodium methane arsonate) Cacodylic acid (Dimethylarsonic acid)
10
Manual method Automated methods Nitric-sulNo Ultraviolet Acid-persul- furic acid digestion radiation fate digestion digestion
5 10
3.9 2.8 1.8
9.8 4.8
15
7.9
10
6.3
14.9 10.4
9.8 5.0 7.0
2
14.4
10 7.5
5.6
10.2
1 5
Based on several replicates. FLOW mLlmin
TUBE D l A
Table 11. Comparison of the Automated and Manual Determination of Arsenic on Filtered Water Samples (All results in pg/L)
ins.
1
I
Manual
(
< 1o-turn
COll
20-turn
COI
20-turn
COII
,
0 42
0 035 Sodium h y d r o x i d e
0 42
0 035 POtasslUm
3 90
0 I10 Sodlum b o r o h y d r i d e
3 90
0 I10 Hydrochlorlc
iodlds
I 1
I
delay coil
wash
acid
I
water
0 110
reCeDloelE
PROPORTIONING PUMP
Figure 1. Acid persulfate digestion manifold. If only dissolved arsenic
is to b e determined, add air just prior to the sulfuric acid
t o obtain a constant temperature of 800 "C. Use a portable pyrometer to check the temperature of the tube furnace. Place the tip of the thermocouple just inside the end of the tube. Remove the thermocouple when the furnace temperature becomes constant. Set up the manifold as described in Figure 1 (acidpersulfate) or Figure 2 (ultraviolet radiation) (use only for dissolved arsenic samples that have been filtered). Feed all reagents through the system using demineralized water in the sample line and allow the heating bath to warm to 95 "C. Beginning with the highest standard, place a complete set of standards in the first positions of the tray. It is best to place two or three samples of the highest standard at the beginning since the first peak is usually low. (No explanation can be given for this phenomehon.) The first standard should not be used in any of the calculations. Remove the sample line from the demineralized wash solution when the baseline stabilizes and begin analysis. For bed materials, weigh 100 mg or less of sample, transfer into a 100-mL volumetric flask, dilute to volume with demineralized water, and analyze as above., When the analyses are complete, the tubes in the manifold and separator column should be cleaned if any sediment is present. The cleaning is accomplished by circulating 6 M hydrochloric acid and demineralized water through all the tubes.
RESULTS AND DISCUSSION Two automated digestion procedures are incorporated into the above described arsenic methods: (1) ultraviolet radiation and (2) acid-persulfate digestion. Both procedures degraded the organic arsenic compounds, in demineralized water matrix, to about the same degree (Table I). Without digestion, recovery of arsenic from these compounds was low. The arsenic recovery was also low when the standard nitric-sulfuric manual digestion procedure was used. The loss of organic arsenic compounds in the standard nitric-sulfuric acid digestion method cannot be explained. It was first thought that 1600
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
Automated methods Acid-persulfate Ultraviolet digestion radiation
Sample No. 1
0.4 0.7 1.0 1.1
2 3 4
5
1.2
6 7
1.6 1.9
9
3.2 5.1 5.3
10 11 12 13
6.4 8.3 8.7 10.0 18.2
14 15 16
sulfuric acid digestion
0.4 1.4 0.7 1.0 0.8 1.1 0.4
1.6
8
method Nitric-
0.0
0.3 0.4 0.8 0.8
1.2 0.0
1.6 3.2
3.0
4.7 4.8
6.0
6.0
4.9
7.6 7.5 9.9 18.1
6.0
1.7
1.0
9.0 9.0
17.0
Table 111. Arsenic Working Curves for Automated Methods
Arsenic standards, pg/L
Recorder peak heights in absorbance mode Acid perUltraviolet sulfate radiation digestion
15.0 12.5 10.0 5.0
58 51 40
2.0 1.0
9 5
21
58 49 39 21 9
5
the organic arsenic compounds were partially volatilized during the manual digestion process, whereas in the automated processes there could be no loss because the compounds were confined in a closed system. T o determine if this were the case, the same organic arsenic compounds as well as inorganic arsenic were digested to fumes of sulfur trioxide in distillation apparatus and the distillates were collected in both weakly acidic and basic solutions. The concentrations of arsenic found in the distillates were negligible. Recoveries of arsenic in the digests, except for the inorganic arsenic solutions, were similarly low as is shown in Table I. It appears that the arsenic has been converted to some unreactive form in the digest. Further investigation is warranted and planned.
~~~~
~
Table IV. Precision of Automated Methods on Standard Reference Water Samples (SRWS) (All results in pg/L) Sample No.
Mean
Std dev
SRWS 52 SRWS 53 SRWS 56 SRWS 57
9.1 55.5 14.3 5.4
3.7 12.8 4.9 1.3
Acid-persulfate digestiona Range Mean Stddev 9.5-1 1.0 55.0-64.0 14.6-16.6 5.3-6.3 4.6-5.1
DC
10.4 59.6 15.5 5.8 4.8
Ultraviolet radiationb Range Mean Std dev
0.5 2.8 0.7 0.3 0.2
9.0-1 1.2 5 5.5-66.0 13.2-14.5 4.3-5.3 4.3-5.2
10.4 61.4 13.8 5.0 4.7
Values based on four replicate determinations. a Values based on seven replicate determinations. contains 5.0 uc/L of arsenic from Dhenvlarsonic acid.
1.0 4.4 0.6 0.5 0.4
Sample solution
Table V. Recovery of Organic Arsenic Compounds in Water-Suspended Sediment Mixtures (All Results in pg/L) Sample No.
Amount presenta
1
3.2 1.4 1.0 0.8 3.1 1.4 1.0 0.8
2
3 4 1 2 3 4 a
Amount added
Founda Ultraviolet radiation
Total
10 (as phenylarsonic acid) 10 (as phenylarsonic acid) 10 (as phenylarsonic acid) 10 (as phenylarsonic acid) 10 (as cacodylic acid) 10 (as cacodylic acid) 10 (as cacodylic acid) 10 (as cacodylic acid)
13.2 11.4
9.3 7.4 8.2 7.7 12.6 9.9 10.3 10.6
11.0 10.8 13.1 11.4 11.0 10.8
13.3 10.2
11.0 9.8 12.0
10.0 10.1 10.2
Based on dwlicate values of both automated Drocedures. Table VI. Comparison of Automated and Manual Analyses on Water-Suspended Sediment Mixtures (All results in pg/L)
A
U L T R A V I O L E T DIGESTOR
2 0 - tYl"
d
-
Acid-persulfate digestion
S4MPLER I V 30/hour 1/2 corn
I20
0 0 5 6 A1r
042
0035 Potassium i o d i d e
3 90
0 I10 S o d i u m borohydride
Sample
... ... ... ...
COll
2 0 - t u r n COll T o SAMPLER I V 40- f t wash Time delay c o l i receptacle To s e p o r o t o r ~
I $W - oetr
390
0110
I PROPORTIONING
PUMP
Figure 2. Ultraviolet radiation digestion manifold
Table I1 shows results found for arsenic on filtered natural water samples (0.45-km membrane filter) by both automated and manual methods. There is excellent agreement between results found by the ultraviolet radiation and acid-persulfate automated methods. The results generally agree with those found by the manual nitric-sulfuric acid digestion procedure followed by manual generation of the hydride and measurement by atomic absorption. The almost identical and linear arsenic working curves for the two automated methods based on replicate analyses are shown in Table 111. Table IV gives the precision and an indication of the accuracy of the automated methods. Several Standard Reference Water Samples used by the Geological Survey and one organic compound, phenylarsonic acid, were used for this purpose. The precision of both digestion procedures is about the same. The method for water-suspended sediment mixtures and bed materials (less than 2 mm) incorporates only the acidpersulfate digestion. The study showed that recovery of arsenic from organic compounds was lower when ultraviolet radiation was used (Table V). This is caused by the loss in penetration of ultraviolet radiation in the samples in which suspended material was present. This was especially apparent
Automated methods UltraAcid-persulfate violet digestion radiation
3.1 3.2 10.0 18.0 143
0.8 1.0 1.4 3.1
... ... ...
... 28
Manual method Nitric-sulfuric acid digestion 1.0 2 2 5 2 4 7 15 140
for sample 9 in Table VI. The quantity of suspended material was also a problem in the acid-persulfate method. Samples containing sediment concentrations above 1 g/L caused various problems. First, sediment collected at various points in the manifold; second, the baseline drifted considerably and arsenic peaks became very ragged. It is suggested that the ultraviolet radiation procedure be used if only dissolved arsenic is to be determined; however, if a laboratory is also analyzing water-suspended sediment mixtures and bed materials, the acid-persulfate method should be used. This would eliminate the need of converting the manifold from one system to another or from having a separate manifold, for dissolved arsenic. The quartz tubes have a limited life. Over a period of time, the voltage applied to the tube furnace was increased gradually in order to maintain a temperature of 800 O C . When it became impossible to obtain a temperature of 800 "C with the voltage regulator, the tube and nickel-chrome wire were replaced. An interference study was not conducted since the arsine is freed from the original sample matrix. Also, a detailed inorganic interference study by Pierce and Brown (8) showed that most trace elements below 300 Kg/L do not interfere. ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1601
LITERATURE CITED (1) P. D. Goulden and P. Brooksbank, Anal. Chem., 46, 1431 (1974). (2) F. D.Pierce, T. C. Larnoreaux, H. R. Brown, and R. S. Fraser, Appl. Specfrosc., 30, 38 (1976). (3) R. S.Brarnan and C. C. Foreback, Science, 182, 1247 (1973). (4) Y. Talmi and D. T. Bostick, Anal. Chem., 47, 2145 (1975). (5) J. S. Edrnonds and K. A. Francesconi, Anal. Chem., 48, 2019 (1976). (6) A. A. El-Awady, R. B. Miller, and M. J. Carter, Anal. Chem., 48, 110 (1976).
(7) P. D. Goulden, 8.K. Afghan, and P. Brooksbank, Anal. Chem., 44, 1845 (1972). (8) F. D. Pierce and H. R. Brown, Anal. Chem., 48, 693 (1976).
RECEIVED for review May 2, 1977. Accepted June 29, 1977. The use of the brand name in this report is for identification endorsement by the u.s* purposes Only and does not Geological Survey.
Determination of Ethylene Oxide in Gas Sterilants by Fourier Transform Infrared Spectrometry P. V. Allen and A. J. Vanderwielen" Control Analytical Research and Development, The Upjohn Company, Kalamazoo, Michigan 4900 1
A quantitative infrared determination of ethylene oxide in dichlorodifluoromethane has been described. The method was developed on a computerized single beam Fourier Transform Infrared (FTIR) system to overcome some of the problems associated with dispersive infrared spectrometers. The FTIR method gave accurate resuits between 5 and 25 %, calculated on a weight basis. A comparison was made between a gas chromatographic method, an infrared analysis on a dispersive infrared spectrophotometer, and the FTIR determination. A brief study on difference spectrometry has been made and showed promise as a quality control technlque.
Gas sterilization using ethylene oxide or propylene oxide as the active ingredient has become a preferred method for treating a wide variety of foodstuffs, drugs, and medical supplies and equipment. Basically, gas sterilization is used because certain materials cannot be readily sterilized with dry heat, steam, or chemical soaks without damage (1, 2). Present analytical methods utilize gas chromatography to determine the amount of active ingredient present in the sterilizing gas mixture. Gas chromatography using a flame ionization detector has been investigated (3-5) and satisfactory results were obtained by using a gas-sampling loop as verified by two independent methods ( 5 ) . However, a large fraction of some gas sterilants consists of halocarbons (e.g., CClzFz is a common inert propellant) and flame ionization detectors are not well suited for halocarbon analysis. T o date, there have been no studies that have examined the possibility of utilizing infrared spectrometry as an analytical method to identify and quantitate the amount of ethylene oxide in gas sterilants, although infrared analyzers have been used to monitor the amount of ethylene oxide in sterilization units ( I ) . The objective of the present research was to examine the viability of using infrared spectrometry as an analytical method for determining the ethylene oxide content in dichlorodifluoromethane (Freon 12) in cylinders of sterilizing gas using Fourier Transform Infrared spectrometry. In this paper we describe a quantitative infrared method for determining the ethylene oxide concentration in cylinders of sterilizing gas containing ethylene oxide and Freon-12. Procedures for taking samples from the gas cylinders and analyzing these gas mixtures have been established. The results are compared to quantitative methods using a con1602
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
ventional double-beam grating spectrophotometer and a n analytical gas chromatograph. We also report the results of a brief study to determine the usefulness of difference spectrometry as a quality control method (6).
EXPERIMENTAL Infrared. Spectra were obtained on a Digilab Model FTS-1OM Fourier Transform Infrared Spectrophotometer (FTIR) and on a Perkin-Elmer 421 grating spectrophotometer. The P-E 421 was set to scan at approximately 9 cm-'/s using a fivefold gain and no suppression. These conditions give a nominal resolution of about 6 cm-'. Calibration curves were constructed using standard mixtures which were prepared in a vacuum line by combining pure Linde ethylene oxide and dichlorodifluoromethane. Spectra measured on the FTS-1OM were collected in a single beam mode and ratioed against a stored spectrum of the empty gas cell. The spectrophotometer was purged using dry nitrogen. From 50 to 500 scans were cc-added to obtain an acceptable signal to noise ratio at 4 cm-' nominal resolution (as given by the FTS-1OM instrument settings). All calculations (except one as will be indicated later) were carried out in single precision without apodization of the interferograms. Except for the difference spectra obtained using the absorbance subtraction technique, spectra were plotted on the transmittance scale and the intensities of the peaks were measured by the baseline method. This worked very well since the empty gas cell provides the ideal reference and flat baselines were obtained for the ratioed spectra. Gas Chromatography. The sterilizing gas mixture was also analyzed by gas chromatography (GC) using a Hewlett-Packard Model 402 instrument with a flame ionization detector. The GC conditions used were those adapted by P. B. Bowman and P. A. Hartman (7) for headspace analysis (8)and are as follows: (i) Column: 4 f t X 0.25 inch 0.d. glass, packed with Porapak R; (ii) Temperatures: Oven at 67 "C, injector at 64 OC, detector at 120 "C; Carrier Gas: Helium at 40 mL/min. A 100-pL gas tight syringe was used to withdraw samples from the cylinder (method discussed under Sampling Procedures) and inject approximately 30 pL of gas onto the GC column. Sampling Technique. We attached a two-stage regulator to the tanks of sterilizing gas and assembled the sampling apparatus shown in the diagram depicted in Figure 1. The regulator was adjusted to deliver 5-10 psi of gas to the 250-mL sampling bulb. Special care must be taken in sampling to prevent fractionation of the gas mixture. The sampling apparatus was purged six times in a stepwise fashion so that there was never a dynamic flow through the system which could cool the regulator enough to cause fractionation (whenever practical, a heat gun or heating tape should be used to keep the regulator at or above room temperature). The sample was then trapped in a small bulb on a vacuum system using liquid nitrogen, warmed to ambient
