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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Chemiluminescence Detector for the Measurement of Nickel Carbonyl in Air D. H. Stedman," D. A. Tammaro, D. K. Branch, and R. Pearson, Jr. Departments of Chemistry and Atmospheric and Oceanic Science, University of Michigan, Ann Arbor, Michigan 48 109
The measurement of ppb (by volume) levels of Ni(CO), in air has been accomplished using a reaction with ozone and purified carbon monoxide. Optimization studies of the emission spectra, and the effects of flow, pressure, and interference have resulted In a portable detection unit using CO modulation. This detector has a 2-ppb detection llmit while operating continuously ai atmospheric pressure. Design criteria are determlned for yet more sensitive units.
Nickel tetracarbonyl is a toxic compound (1). Although its principal industrial use is in refining high purity nickel (2, 3), i t may also be produced as a byproduct of other industrial processes (1,4 ) . A threshold limit value for this compound has been set at 1 ppb ( 5 ) ,so a monitor for airborne Ni(C0)4 must operate at this level with fast response and portability for industrial situations. Many sensitive methods for determining Ni(C0)4 have been developed. Brief e t al. ( 4 ) , and Densham e t al. (6) have reported procedures for chemical collection and analysis. Plasma chromatography has been used by Wernlund and Cohen ( 7 ) and infrared spectrometry by McDowell (8). Mantz (9) has applied Fourier transform infrared spectroscopy t o Ni(C0)4 in CO at pressures below 1 Torr (130 Pa) but did not report measurements in air or from samples with ppb level concentrations at atmospheric pressure. To our knowledge, none of the methods currently in use combines sensitityity, speed, and portability, although the GC method of Sunderman e t al. (10) can be made sufficiently sensitive. A chemiluminescent method for Ni(C0I4 based on the studies of Groth e t al. (11) has been developed to meet this need (12) after pioneering work by Morris and Niki (13). The first version of the instrument achieved sensitivity in the part-per-trillion (ppt) range with a fast response time (12). B u t strong interferences were encountered with Fe(CO)5 and NO, and the carbonyl signal intensity was a function of sample humidity. This unit operated under vacuum, and by virtue of its weight and power requirements was intrinsically nonportable. T h e flow/pressure studies and spectroscopic studies described below were undertaken to determine whether a chemiluminescent detector could operate a t atmospheric pressure without interference. These results have been employed t o design a portable instrument with significantly reduced interference, high sensitivity, and good response time.
EXPERIMENTAL Mixtures of Ni(C0)4, Fe(CO)5, or NO in air were sampled through an orifice into a flow system and mixed with purified CO. This stream was allowed to react with ozonized O2 in a 150-mL volume, and chemiluminescence was observed through a Pyrex window. The spectra in Figure 1 were obtained with a 1-m Czerny-Turner scanning monochrometer using 10&300 ppm of Ni(CO).,, Fe(COI5,or NO, 100-300 ppm 03,and 1%CO at a total pressure of 1-5 Torr (130-670 Pa). Comparison with the tabulation of Pearse and Gaydon (14) indicated that all peaks from the samples containing Ni(CO)4and Fe(CO), correspond to known 0003-2700/79/035 1-2340$01.OO/O
Table I. Chemiluminescent Signal in nA Corresponding t o a Concentration of 10 ppb at a Pressure of 1.5 I0 . 5 Torr (200 r 7 0 Pa)" sample gas green filterb red filterC
Ni(CO), Fe(CO), 69 68
0.25 14
NO
l x 10 'l 3.6 x 10
a The flow rates for sample, CO, and 0 1 / 0were , 150 mL min-I, 100 mL min-', and 30 m L min-I, respectively. The PM tube (type HTV R374) noise was 0.03 nA at 0 'C. A 2-in. diameter, 10-nm bandpass interference filter centered at 492 nm. A cutoff filter passing wavelengths h > 600 nm.
Table 11. Chemiluminescent Signal Intensities Relative to Ni(CO), Using Red and Green Sensitive Photomultiplier Tubes" sample gas
Ni(CO),
Fe(CO),
green PM tube
1
0.1
6
1
0.1
2x10
NO
x
(HTV R268)
red PM tube (HTV R374)
a The noise level with a 1-s response time corresponded t o 10 ppb Ni(CO), for the green sensitive tube and 0.1 DDb for the red sensitive tube.
transitions of NiO and FeO, respectively, as shown by Groth et al. ( 1 1 ) . Mixtures of CO (Matheson C.P. grade) taken directly from the cylinder and mixed with 03/02exhibited chemiluminescent spectra indistinguishable from those obtained from mixtures of Fe(CO),, CO, and 03/02. This was attributed to Fe(C0)5present as an impurity in the CO at a level of 0.7 ppm. Initially this impurity was removed by heating the CO to 460 "C in a Vycor furnace, although this proved erratic, possibly owing to subsequent recarbonylation of decomposed material. Subsequently a 50-mL trap consisting of a few crystals of I2followed by 8-mesh activated charcoal to remove excess gaseous I2 in a gas scrubbing tube has proved effective and more convenient than the furnace. At room temperature the 12/charcoal trap reduced the level of contamination to below 0.3 ppb. Attempts to observe a spectrum from the fuel additive methyl cyclopentadienyl manganese tricarbonyl by passing a stream of air over the liquid at 25 "C and adding CO and O3 were unsuccessful, although MnO bands would be expected in the visible region. A transient chemiluminescent signal observed at the start of the experiment was attributed to volatile impurities present in the original sample. Apparently the liquid is sufficiently involatile that any gas phase concentration of this species is below the detectability limit of the apparatus. The specificity of the chemiluminescent detector can be significantly improved by matching the response of the filter and PM tube to one of the spectra in Figure 1. Replacing the red filter customarily employed in NO detectors with a green interference filter results in much better rejection ratios for Fe(COI5 and NO in Ni(CO), measurements as can be seen from Table I. The response of red- and green-sensitive PM tubes (HTV R374 and HTV R268, respectively) to these species is shown in Table 11. Both tubes were cooled to 0 "C and used without filters. Although signal levels are comparable, the signal-to-noise ratio B 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO 14, DECEMBER 1979
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be expected to exhibit a pressure dependence of the form I = A ( l - exp(-BP)) / ( C DI')
+
where P is the pressure and A-D are appropriate constants. The exponential term gives a decrease in light at low pressures, as the reaction becomes too slow for much light to be emitted in the reaction chamber. The linear term, (C + DP), decreases light intensity at high pressures because of competitive reactions and quenching. Qualitatively these effects are observed. Thus this equation can be rearranged to give
I-' = (C + D P ) / A ( l
Figure 1. Uncorrected chemiluminescent emission spectra from high concentrations of NO, Fe(CO),, and Ni(CO), with CO and 03/0,
=>.
I I
a
I00 100
IS0 50
.
I
(z
t
2
40
H
PRESSURE ( m m Hg)
Flgwe 2. Reciprocal of photocurrent vs. reactor pressure at a constant 03/02flow rate of 30 mL min-' and Ni(CO), concentration of 10 ppb. The lines are smooth curves connecting the data points
is two orders of magnitude better with the red-sensitive tube owing t o increased overlap with the red emission tail. Figure 2 shows the reciprocal of the photocurrent vs. reactor pressure for different flows of CO and Ni(C0I4in air at an 02/03 flow rate of 30 mL min-'. The data were obtained by throttling
the vacuum pump and measuring the pressure in the reaction chamber with a silicone oil manometer. DISCUSSION T h e outline of a possible mechanism can be written ( I 1 13)
+ + + -
O3 + Ni(C0)4
+ products NiO CO Ni + CO, Ni 0, NiO* + 0, NiO* NiO + hu NiO* M NiO + M NiO
(1) (2)
(3) (4)
(5)
It is assumed that O3 and CO are present in excess. The full mechanism of ozone attack on Ni(CO)* is not known ( I I ) , however ground state NiO production is suggested. This mechanism, similar to that of Morris and Niki (13) qualitatively explains the major features of the system: (a) The spectroscopically observed species is NiO* with an excitation energy of -57 kcal/mol. Reaction 3 with an exothermicity of 76 kcal/mol can provide the needed energy. (b) The signal enhancement of approximately a factor of 100 with added CO implies t h a t CO must somehow be involved in formation of the emission precursors. Reaction 2 is postulated to account for this behavior. When H, or SO2 was used in place of CO, no enhancement in signal intensity was observed. By analogy with similar systems, such as NO + 0, ( I s ) , chemiluminescent intensity from the above mechanism can
-
exp(-BF'))
which predicts a linear relation between I-' and P a t sufficiently high pressures. The data given in Figure 2 do exhibit this behavior. More significantly, the small pressure dependence observed in the lowest curve, which corresponds to the largest signal intensity, implies that an instrument operating a t higher pressures than a few Torr can be built with a suitable choice of flow parameters, in particular enabling one to eliminate the ozone sensitive oil sealed rotary vacuum pump. C O Modulated Detection of Ni(C0)4. The addition of CO markedly increases chemiluminescence from carbonyls, but has essentially no effect on the intensity arising from NO. Consequently, a pulsed CO flow can selectively modulate chemiluminescence from Ni(CO), or Fe(CO)5in the presence of NO. This has been implemented by switching the CO flow on and off with a solenoid valve a t a frequency of 0.05 to 0.5 Hz with a 50% duty cycle. Signals a t the modulation frequency were selectively amplified and detected. Since signals from NO were predominantly dc, they were strongly rejected. Other variables affecting NO chemiluminescence which flow rates, and the reaction chamber include sample and 03/02 pressure must be carefully controlled. Large transient chemiluminescent signals from carbonyls have accompanied the CO flow switching under some experimental conditions. They appear to be caused by fluctuations in the reaction chamber pressure and are minimized when the pressure is constant. A portable instrument capable of selectively detecting Ni(Coldin air in the range 2-100 ppb using the CO modulation technique was built by modifying a chemiluminescent analyzer (Thermo Electron Corp. model 8A). As received, the unit weighed 14.7 kg, measured 25 cm x 21 cm x 41 cm, and required 225 W of external power. The ozonizer used dried room air. The modifications required included the following: (1) The red filter between the photomultiplier tube and the reaction chamber was replaced with a Pyrex window 2 mm thick. (2) T h e sample flow capillary was replaced by a larger capillary (0.025-in. bore) to increase sample flow into the instrument. (A set of capillaries of various sizes was supplied with the instrument.) ( 3 ) The ozonizer was connected directly to 110 V ac eliminating the 0.5-Hz pulsing circuit. (4) CO (CP grade) cleaned as described previously was introduced to the reaction chamber along with sample and ozone. A normally closed solenoid valve was incorporated in the CO line and connected to the 0.5-Hz ozonizer pulsing circuit. Thus the CO flow and Ni(CO), signal enhancement were synchronous in the electronics of the instrument. (5) Output terminals for a 10-mV strip chart recorder were installed. (As received, unit had a meter for output.) The instrument was then found to have the following characteristics: H o b Rates. The sample flow to the instrument typically was 2.2 L min-' but operation a t lower flows with reduced sensitivity was possible. The sensitivity to Ni(CO), increased linearly with average CO flow rates up to 500 mL min-'. T o conserve CO, most of the work was done with an average CO flow of 100 mL min-'. The actual flow was twice this for 1
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
apb hi (COI,
Figure 3. Photocurrent in arbitrary units vs. [Ni(CO),] for various humidities for the portable detector before insertion of the permeation drier. DRY corresponds to
