118

In the case of Pb, Zn, Nl, and Fe it was found satis- factory to plot element concentration d~rectly against re- corder values and straight hnes were obtained.

For tin, especially in large amounts (to 11%) it was found necessary to make a correction for variation in the mternal standard other than that provided by the integra- tion itself. Thus, the analytical curve (Figure 1) relates recorder values to % Sn /% Cu. Percent Sn may then be calculated from % Sn -~ % Cu (% Sn /% Cu) , where % Cu is obtained from an electrolytic determination, or calculated from: % Cu -~ (100 - - % Pb - - % Zn, etc.) /1 + (% Sn /% Cu).

Analytical curves once prepared are checked periodic- ally, and daily standardization of the instrument accom- plished using an appropriate calibration standard.

The elements studied, ranges, and precision are shown in Table II. The ranges indicated are those encountered in production of a wide range of copper base alloys. The ex- tension of these ranges and addition of other alloying ele- ments may be feasible and wdl be considered in later studies.

Conclusions

The analytical technique described offers a useful pro- duction control method for analysis of copper base alloys in the element ranges described (over 5000 element deter- minations have been made in our laboratory in recent months). Widely varying alloys can be handled in the same programming sequence and a sample analyzed in a few minutes' time. The method as described represents an ex- tension of maximum limits of element concentration over previously pubhshed work consistent with acceptable ac- curacy.

T

A P P L I E D SPECTROSCOPY

TABLE II. PRECISION AND R A N G E OF E L E M E N T S

Element

Error, % Standard of Amount

Line, A Range, ~ Deviations a'b Present

Cu 3108 6 Int . Std. - - - - Sn 2840.0 ~ 1.0 - 11.5 0.060 1.3 Pb 4057.8 0.05 - 8.0 0.075 1.4 Zn 3345.0 0 .10- 6.0 0.030 0 6 Nt ~1414.8 0 05 - 1 0 0 015 1.5 Fe 2739.5" 0.01 - 0.5{) 0.0085 4.7

'S tandard dewat~on calculated for NBS 124d; % Cu- -83 60, % Sn • - -4 .56, % Pb--5 .20 , ~ Nx--0.99, % Fe--0.18. Comparable results obtained on other bronze and brass alloys.

h Std. dev. calculated from eleven samples, each run four times and averaged.

Second order

Literature Cited

(1) A. W. Young, STEEL, 134, No. 24, 112 (1954)

(2) C. E. Harvey, Spectrochemwal Procedures, Applied Research Laboratories, Glendale, Cal., 1950

(3) SPECTROGRAPHERS N E W S L E T T E R , 21 No. 9 ( 1 9 4 8 )

(4) P. T. Beale and D. A. Poynter, METALLURGIA, 52, 253 (1955)

(5) J. P. Pagliassoti, APPLIED SPECTROSCOPY, 9, 153 (1955)

(6) M. Pierucci and L. Barbantl--Silva, N u o v o C~MENTO 17, 275 (1940); C. A. 36, 6898.6 (1942)

(7) E. V. Rouir and A. M. Vanbokestal, SPECTROCHIM. ACTA, 4, 330 (1951)

Submttted July 18, 1961

An Appearance Potential Recorder V. J. Caldecourt

Chemical Physics Research Laboratory, The Dow Chemical Company, Midland, Michigan

Abstract

A c~rcmt has been developed that records the electron acceleratmg potentml reqmred to generate a barely detectable 1on current at each mass m a mass spectrum. The level of electron accelerating potential attained for each ion peak approaches the true appearance potential for the instrument when the mn peak *s one which would be large w*th 70 v electrons The device is useful to select operating condmons for analyses at low electron accelerating potentials.

Introduction

T h e use of low voltage electrons to excite mass spectra has proven to be a useful analytical tool. I t permits one to suppress interfering ions arising from higher ionization potential compounds and fragments. The method was first used to distinguish saturated from olefinic hydrocarbons. Since then it has been applied to many other analytical problems.

When applying the method to a variety of compound types, determination of the electron accelerating potential, which will produce the maximum number of the desired ions and a minimum number of the interfering ions, re- quires considerable time with the usual recording equip- ment, since repeat scans of the spectrum must be made at

small increments in electron accelerating voltage. In many cases the electron energy chosen for the analytical run can be considerably greater than the appearance potential be- cause the ion abundance of the interfering species is small.

A continuous record of the electron accelerating volt- age, which will just reduce the ion current at each mass to zero, will give the appearance potential &rectly, subject of course to the instrumental bias. Because of limitations in measuring small ion currents and instrumentally con- trolling at zero output, only an approximation to this can be obtained by use of an automatic measuring system. This approximation will be poor for small peaks and better for large peaks.

By designing a circuit that wdl automatically reduce the electron accelerating voltage to that value needed to restrict the ion currents to some small value distinguish- ably above the basehne noise, a record of the approximate appearance potential can be secured. To do this, a feed- back loop between the ion current amplifier and the elec- tron accelerating voltage supply is estabhshed through an operational amplifier. The electron energy is then auto- matically reduced until the ion peak becomes very small.

VOL. 16, No. 4, 1962

tON ELEOTROMf.TER COLLECTOR AMP

PHIL OglOg U S A - 3

..2_

FIG. 1. THE APPEARANCE POTENTIAL RECORDER SCHEMATIC

ION

SOURCE

i F IL I

I

l~ +-

A recording potentiometer measuring the electron acceler- ating potential developed by the amplifier during the scan provides a record of the appearance potential for each mass peak.

Description of the Circuit A schematic diagram of the feedback loop is shown in

Figure 1. The dc electrometer amplifier has an input re- sistor of 2 X 101° ohms and an overall voltage gain of 20. The Phllbrick, chopper stabdlzed dc amplifier has an overall voltage gain of approximately 500. The input time constant is 10 ° ohms x 5 x 10 .9 fd or 5 sec. The tong time constant was used to reduce noise and obtain overall stability of the feedback loop. A scan rate of 4 masses/rain. was used in taking the following data. The combined gain is 10 4 and therefore, 1 mv or 5 x 10 -14 amp of ion current will change the output voltage by 10v.

The output from the Philbrock amplifier is fed thru a 10,000 ohm resistor to a diode limiter circuit. When the ion current is zero, the output is - -40% and this is the electron accelerating voltage. This output is obtained by biasing the Phdbrick dc amplifier. Upon collecting positive ions the output goes positive, and the electron accelerating voltage is reduced. The 1N1763 prevents the output from going excessively positive which would shut off the elec-

FIG. 2. VARIATION O17 ION CURRENT WITH MASS NUMBER FOR D-PRoPYLBENZENE

119

FIG. 3. VARIATION OF ELECTRON ACCELERATING VOLTAGE WITH MASS NUMBER FOR n-PROPYLBENZENE

1000

I O C

fl - P R O P Y L B E N Z ENE

MASS I g O

g

O I | I I ] I I I I I I I I I I 0 2 4 6 8 I 0 12 14 16 1 8 2 0 2 2 2 4 2 6 - 2 8

E L E C T R O N A C C E L E R A T I N G V O L T A G E

FIG. 4. VARIATION OF PEAK HEIGHT WITH ELECTRON ACCELERATING ~/OLTAGE FOR n-PRoPYLBENZENE (MAss

120)

120 AI~PLIED SPECTROSCOPY

tron beam and cause the filament regulator to burn out the filament. The 1N1358 is a Zener diode that will conduct when the output is more than 20v negative and effectively reduce the gain by a factor of ten. This is desirable since the high overall gain of the system when in the region of higher ionization efficiency makes the output, the electron accelerating voltage, a greatly magmfied record of the baseline noise. Recording the magnified noise serves no purpose and increases recorder wear. At low electron accelerating voltages maximum information is desired to obtain the best value for the appearance potential, so the full output is used. A constant 5v potential is supplied to maintain the electron beam when the output of the Phil- brick amphfier is zero. The feedback loop is completed through the ion beam. As shown, our mass spectrometer op- erates with the ion source grounded which simplifies the interconnections to the amplifiers.

Operation

Figure 2 shows part of the mass spectrum of n-propyl- benzene with the circuit in operation. The peaks heights are held to 1 mv or less. At normal ionizing potentials, the peaks at mass 120 and 105 would be several hundred mv high. The sharp spike is a transient due to overshoot on the leading edge of the peak and does not appear in the electron accelerating voltage record.

A record of the electron accelerating potential made on the same sample shown in Figure 2 appears in Figure 3. Note that the electron accelerating voltage goes through a minimum at the point corresponding to maximum peak

height in a normal record. The minimum value for the mass 120 peak is 7.4 v, and for the mass 121 peak it is 9.2 v. The mass 120 1on xs formed from C912H12 . The mass 121 ion results from the presence of naturally occurring C 13 in the same molecular structure giving the formula Ca~Cs12Ha.,. Since these are both molecular ions of n- propylbenzene, the ionization potentials should be the same. The high value for the mass 121 peak demonstrates the lack of leverage resulting from the smaller abundance of that type of molecule. Note that the appearance potential of the mass 118 peak is nearly as low as the 120 peak showing the presence of ,x-methylstyrene.

A plot of the peak height versus electron accelerating voltage for the mass 120 peak of n-propylbenzene is shown in Figure 4 on a semi-log scale. The projected curve indi- cates the ionization potential to be 7.0 v, which is reason- ably close to the value given by the appearance potential recorder for the mass 120 peak.

This simple appearance potential recording system thus gives a relatively rapid survey of the approximate appear- ance potential of each peak in a mass spectrum. The value given will approach the true value for peaks which are large when excited by 70 v electrons. The record does give directly the electron accelerating voltage one should select to suppress certain peaks. For example, in Figure 3 it is apparent that operation at 8 v would eliminate all but the mass 118 and 120 peaks. This is the type of information needed to establish operating conditions for an analysis by "low-voltage" mass spectrometry.

Submitted July 28, 1961

The Determination of Lead, Copper and Zinc In Wines by Atomic Absorption Spectroscopy

P. B. Zeeman and L. R. P. Butler*

Department of Physics, University of Stellenbosch, Republic of South Africa

Abstract Methods are described for the determination of Pb, Cu, and Zn

m wine products. A lead lamp with demountable cathode xs described as well as a new type of burner with an e,ght m. absorbing path and a slotted top. The temperature and hexght of the flame could be adlusted by means of an extra a*r current fed mto the burner through the ordinary acetylene nozzle of a Ze~ss atom*zer. Heated a i r w a s used to atomize the solutions, and the eflic*ency of the burner was approx*- mately 22%. Concentrated H..SO4 was used to dehydrate the samples before ashmg at 500°C Recovery tests for lead were carried out at vamous temperatures The results for lead are compared for a number of samples w~th thole obtained by chemical analyses. S~mllar work ~s descr*bed for Cu and Zn. The coe~clent of variation for Pb, Cu, and Zn was respecnvely 3.7%, 4.23~o, and 4.11~. The results for Cu and Zn for a large number of wine samples are given.

Introduction

The accurate determination of heavy metals in wine products is of the utmost importance to the wine indus- try. Government regulations in many countries fix the maximum allowable concentration of certain elements. Wet chemical methods do exist to determine these elements, but they are often time consuming and difficult. The recent

Present Address: National Physical Research Laboratory, Council for Scientific and Industmal Research, Pretona, South Africa

development of atomic absorption spectroscopy (1, 2) has presented a simple and accurate means for analysis, which could be applied to this field. The determination of lead in wines by atomic absorption spectroscopy has already been described (5), but further work has been done to improve and simplify the apparatus used and also to ex- tend the method to include the analysis of Cu and Zn in wine products.

The Determination of Lead The Hollow Cathode Lamp

The hollow cathode lamp which was used for this invesngatlon is shown in Figure 1 and was developed in- dependently. In a later publication Jones and Walsh (4) submitted very useful information about the manufacture of hollow cathode lamps. The anode, 1, and the cathode, 5, are housed in a 250 ml Pyrex flask, fitted with a side tube and quartz window for the transmission of ultraviolet light. The anode is a tungsten wire of 2 mm diam., and the cathode is machined from a solid steel cyhnder to the form shown in the figure. The cup portion of the cathode is 3.3 cm deep, and has an tuner diameter of 10.0 mm with a 1 mm wall thickness. The stem of the cathode has a