TRANSLATION1 ~ STAGES ]~

TORCH[ i BOX I

• OPTICAL RAIL

i

TUNING NETWORK

A k 12"

FIG. 3. Top view of t ransla table torch housing, impedance ma tch ing network, and optical table.

6 3 5 mm A B O V E L O A D C O I L

458 - - - H 6 II (2798 rim)

/ ~ - - N6 I (285 2 rim)

368-

>-

L O L 27e-

Z k

~J

~ J8e-

z W

~ 98-

~9 Z

8- I I I ~ I r I l I" I i I - 3 8 - 2 5 - 2 8 - 1 S - l . 8 -8 .5 8 8 85 I 8 1.5 2.8 25

LATERAL TRANSLATION

FIG. 4. Horizontal emission profiles of m a g n e s i u m ion and a tom lines obtained with the new t ransla table torch. Vertical location in plasma, 6.35 m m above load coil. Other spatial profiles similar to those in Ref. 1.

micrometer control stages are not essential in this appli- cation, but offer a convenient means of reproducibly establishing torch location relative to a reference point.

Ignition of the plasma was found to be straightforward using standard operating procedures, typical plasma, aux- iliary and aerosol gas flow rates, rf power settings, and impedance match settings. Moreover, under automatic impedance-matching operation, the presence of the braided cables produced no significant tuning changes.

In order to verify the similarity of the translatable plasma to an unmodified unit, horizontal spatial emission profiles of magnesium obtained at several heights above the load coils in the modified system were compared with emission profiles of the same element from a standard ICP unit) Both neutral atom (285.2 nm) and ion (280.3 and 279.5 nm) lines were investigated with no noticeable

Volume 36, Number 6, 1982

differences between the torch configurations being ap- parent (cf. Fig. 4). The plot of the two ion lines depict the characteristic dip in maximum emission intensity near the aerosol channel.

Radiation leakage from the exposed braided cable was checked as a possible radiation hazard. A radiation haz- ard meter (General Microwave Corporation, Farming- dale, NY, model 2) was used to detect the radiation density at various distances from the braided power cables. At a distance of 1.5 cm from the braided cables, rf power density was found to be less than 5 mW/cm 2. Beyond distances of 3 cm, the rf power was not detectable on the most sensitive scale (2 mW/cm 2 full scale) of the available instrument. Under normal operating conditions the operator is approximately 1 m from the torch housing; even when engaged in manually translating the plasma, he ordinarily remains at least 25 cm from the power cables. Consequently it was judged that no significant radiation hazard exists in the new arrangement.

The removal of the torch box from the matching network and its subsequent mounting on commercially available translation stages permits precise and accurate positioning of the plasma. This positioning capability, without the required movement of cumbersome pieces of equipment, facilitates fundamental studies and the rou- tine use of the ICP as a source for analytical spectroscopy.

ACKNOWLEDGMENTS

Supported in part by the National Science Foundation through grant CHE 79-18073 and by the Office of Naval Research.

1. M. W. Blades and G. Horlick, Spectrochim. Acta 36B, 861 (1981). 2. J. P. Walters, in Contemporary Topics in Analytical and Clinical Chemistry,

D. M. Hercules, G. M. Hieftje, L. R. Snyder, and M. A. Eveuson, eds. (Plenum Press, New York, 1978), Vol. 3, p. 91.

Use of an Extended Torch with an Inductively Coupled Plasma for the Determination of Nitrogen in Aqueous Solutions

CONSTANCE BUTLER SOBEL

Bausch & Lomb~Instruments & Systems Division, 9545 Wentworth Street, Sunland, California 91040

Index Headings: Analysis for nitrogen; Emission spectroscopy; Inductively coupled plasma

The inductively coupled argon plasma (ICP) has proved to be an excellent source for determining a variety of elements in various materials. 1 The ICP has been used by several authors to determine nitrogen in various forms but not directly from a liquid sample. Alder, e t al . 2

determined low levels of ammonium by oxidation with

Received 1 February 1982; revision received 22 March 1982.

APPLIED SPECTROSCOPY 691

sodium hypobromite in an alkaline medium. The result- ant evolved nitrogen was then passed into an ICP and the emission of the NH band emission at 336.0 nm measured. Gaseous compounds containing nitrogen were analyzed by Northway e t a l . ~ by injecting samples into the ICP and measuring the 868.027 nm N line. Brown e t

a l . 4 have reported the successful analysis of volatile or- ganic liquids for nitrogen with the use of the ICP as a nitrogen selective detector for gas-liquid chromatogra- phy.

Heine, Babis, and Denton ~ reported that the most sensitive nitrogen lines are found in the vacuum ultravi- olet region of the spectrum. This paper describes a tech- nique for the direct determination of nitrogen in various compounds and materials with the use of an ICP source, a vacuum spectrometer, an argon-purged optical transfer path, and an extended torch configuration. The extended torch was necessary to exclude any entrained air from the plasma.

The Bausch & Lomb model 34000 inductively coupled plasma system, which incorporates a vacuum spectrom- eter, was used for this investigation. Table I shows the experimental and operating facilities. The standard torch with the 34000 system was modified so that the coolant tube was extended 18 mm and a side arm added. This side arm was sealed to the transfer optics so that there is a direct optical path from the plasma to the lens in the instrument. With the side arm opening directly to the plasma, the measured intensities are not dependent on the optical quality of the quartz extension, which may be degraded with time and use. This optical path is purged with 1 L of argon/min. With this flow rate, the coolant flow surrounding the plasma is not disturbed and air is effectively excluded from the transfer optics. The fused quartz lens at the entrance slit has a 78% transmittance at 170.0 nm.

Standards were prepared by pipetting known amounts of nitric acid into volumetric flasks and adding H2SO4 so that the final concentration of the sulfuric acid was 10% by volume. Samples were prepared by dissolution in sulfuric acid and taken to volume so that the final con- centration of the sulfuric acid was also 10% by volume.

The use of the extended torch excludes air from the plasma and allows the determination of nitrogen. A ni- trogen blank of considerable magnitude is still apparent even though air has been excluded from the torch by sealing the nebulizer inlet and the ball joint to the spray chamber. Little of this nitrogen blank is due to nitrogen from dissolved air in the sample, and it is assumed that the argon gas is contaminated. However, no attempt was

TABLE L E x p e r i m e n t a l facilities and operating conditions.

Instrument:

Slits: Nebulizer:

Spray chamber:

Gas flow rate:

Observation height:

Applied Research Laboratories model 34000 Vacuum Quantometer and a rf Generator (27.12 MHz band) operated at 1200 W

15 #m primary slit; 50 ttm exit slits Concentric pneumatic (Meinhard type) with

an uptake rate of 2.2 ml /min All glass conical spray chamber (volume = 90

n~) Plasma coolant argon, 12 L/min; auxiliary

plasma argon, 0.8 L/min; aerosol carrier ar- gon, 1.0 L/min; optical t ransfer argon, 1.0 L/min

Centered 15 mm above the load coil

made to scrub the argon before allowing it to flow into the torch.

Calibration standards for the determination of nitrogen were prepared with nitric acid. This material was chosen as it is readily available, easy to purify, and inexpensive. Since many of the nitrogen-containing compounds dis- solve readily in sulfuric acid, 10% by volume of sulfuric acid was added to the nitrogen calibration standards and samples. This amount of sulfuric acid was also used to compensate for the differences in viscosity of various liquids that contain nitrogen. The use of sulfuric acid has little effect on the detection limit determined for nitro- gen. The detection limits for the nitrogen (174.27 nm line) with and without sulfuric acid present were 27 and 35 ttg/ml, respectively.

The calibration curve for the determination of nitrogen covered the range of 0.2% to 1.4% nitrogen (Fig. 1). Care was taken when the data were collected because of air entrained in the uptake tube of the nebulizer when so- lutions were switched. If 2 min are allowed to elapse after switching solutions before collecting data for both stand- ards and samples, a constant blank is obtained. Pumping of the sample or a technique devised by Ripson and DeGalan 6 could be used to correct for viscosity differ- ences and lessen the time between samples as the amount of air between samples would be lessened considerably.

l e o -

8 0 -

w b- _z

4 0 .

2 0 -

I L I I 0 ,2 0.6 1.0 1.4

CONCENTRATION (O/o)

FIC. 1. Calibration curve for the determination of nitrogen.

TABLE II. Analysis of nitrogen compounds.

Ni- Certi- Calcu- Rela- tro- fled lated Amount tive gen nitro- found nitro- error onla- gen (%) (%) bel gen (%) (%) (~)

Nitrogen compounds Amino acetic acid Aniline Nicotinic acid Potassium cyanate L-glutamide Nickel nitrate

Commercial fertilizers Angel City Terrovite Schultz Spoonit

NBS-SRM Materials NBS Orchard Leaves

0.50 0.51 2.0 0.45 0.46 2.0 0.50 0.48 4.0 0.50 0.51 2.0 0.50 0.49 2.0 0.45 O.48 6.6

4.9 16.7 12.1 18.4

3.0 7.1

5 16 10 18

2.8

692 Volume 36, Number 6, 1982

Several nitrogen-containing compounds, both inor- ganic and organic, were analyzed for nitrogen. The results obtained, as seen in Table II, show that nitrogen can be determined for the first six compounds with an accuracy of approximately 5% of the amount present. The agree- ment was good for the four commercial fertilizers also, however, the values, as reported on the labels, were not certified. The National Bureau of Standards reference material--orchard leaves--had been just charred with sulfuric acid but not completely dissolved.

The precision of the method was determined by ana- lyzing a synthetic sample on 7 different days. The stand- ard deviation for this technique is 2.0% relative at the 1.0% concentration level.

1. R. M. Barnes, CRC Critical Rev. Anal. Chem. 7,203 (1978). 2. J. F. Alder, A. M. Gunn, and G. F. Kirkbright, Anal. Chim. Acta 92, 43 (1977). 3. S. J. Northway, R. M. Brown, Jr., and R. C. Fry, Appl. Spectrosc. 35, 125

(1981). 4. R. M. Brown, Jr., S. J. Northway, and R. C. Fry, Anal. Chem. 53, 934 (1981). 5. D. R. Heine, J. S. Babis, and M. B. Denton, Appl. Spectrosc. 34, 595 (1980). 6. P. A. M. Ripson and L. DeGalan, Spectrochim. Acta 36B, 71 (1981).

Optimization of Stability in the Output of a 1-m Scanning Monochromator

B. A. R. TAIT

Geochemistty and Petrology Division, Institute of Geo- logical Sciences, 64-78 Gray's Inn Road, London WCIX 8NG, England

Index Headings: Emission spectroscopy; Optics.

Instability and fluctuations in the performance of spec- trometers and spectrographs can be caused by several factors, including changes in temperature and pressure, and vibrations due to seismic and other phenomena. The role of air currents in causing instability and fluctuations was recognized some years ago and is mentioned in the literature)' 2

An inductively coupled plasma atomic emission spec- trometer (ICPAES), incorporating a Spex 1802 1-m scan- ning monochromator, is in use in the laboratories of the Geochemistry and Petrology Division of the Institute of Geological Sciences. Some time after installation and initial testing of the equipment, varying degrees of both long- and short-term instability were observed. The long- term drift was found to be associated with the consider- able variations in laboratory temperature (not stabilized), while shorter fluctuations, with a variable periodicity averaging approximately 20 s, were not consistently as- sociated with these thermal variations, but occurred mainly when the temperature was falling or stationary.

Received 30 November 1981; revision received 31 May 1982.

As the long-term instability was almost entirely man- ifested as a wavelength drift, the instrument was unaf- fected when scans of spectral lines were carried out, and this method of measurement was used in the initial use of the system. It was desired, however, to make measure- ments of successive samples with the monochromator stationary on one line, using fairly narrow entrance and exit slits to reduce spectral interferences. Under these conditions the instabilities manifested themselves to such a degree that an immediate and complete solution to the problem was required.

Stability tests were carried out using a copper hollow cathode lamp light source with the zero order line being observed. The zero order line was used because its posi- tion does not depend on the wavelength stability of the light source. Separate measurements of the hollow cath- ode lamp intensity were made using a wide (100 ~m) spectrometer exit slit. The observed steady signal showed that the lamp and detector system were intrinsically stable and that the fluctuations were related only to effects arising from the spectrometer. Wavelength drift of up to 0.05 nm for a 1.0°C change in temperature was observed, whereas short-term fluctuations generally, but not exclusively, appeared when the temperature of the laboratory was falling (Fig. 1) (spectrometer dispersion is 0.8 nm mm-1).

The degree of wavelength drift for given temperature changes was checked by setting the spectrometer (with no insulation or temperature control) on a shoulder of the zero order line (Fig. 2). The entrance and exit slits were both set at 20 #m and the recorder chart speed at 600 mm/h. Measurements were taken in the morning, while the laboratory was becoming warmer. By slowly scanning across the line on a previous occasion the rela- tionship between line profile and wavelength had been established. The ratio of wavelength drift to temperature was calculated from a chart record of line shoulder inten- sity. The data obtained from Fig. 2 indicate a ratio of 0.026 nm/°C. The ratio has been found to vary from approximately 0.01 nm/°C to 0.05 nm/°C, appearing to depend on temperature fluctuations in the laboratory, both at the time of the measurements and for several hours beforehand. The highest ratio is usually observed early in the morning, at a time when the temperature of the laboratory is rising rapidly. The above checks of temperature drift were repeated several times after the temperature stabilization modifications had been made, the temperature control being switched off and the in- sulation being removed for the duration of the experi- ments. Overall stability of the spectrometer was good only when the temperature of the laboratory had been stable for several hours. The manufacturers of the instru- ment considered that the drift might be due to changes in the physical characteristics of the lubricant layer be- tween the sine-bar drive nut and the lead screw. This possibility was carefully checked several times but was rejected because the drift was unrelated to the time elapsed since scanning, and also because of the consistent relationship between temperature and drift. Relubrica- tion of the lead screw had no effect, and the application of slight tension to the sine-bar drive merely reduced the scanning backlash.

It was decided initially to attempt to control the tem-

Volume 36, Number 6, 1982 APPLIED SPECTROSCOPY 693