Quantometry in 1952

SpectrocbimicaActs. 1953. Vol. 6, pp. 69 to

Communxation

i9

Pergamon Press Ltd., London

to the Third International

Spectroscopy

Qnantometry

Co&q&urn,

High Leggh, 1952

in 1952

31. F. HssLER Apphed Research

Laboratories.

Glendale, California

Our company is just completing its eleventh year of development work on direct,reading instruments and methods of spectrochemical analysis suitable for such instruments. It is complet,ing its sixth year of production of Quantometers. of which production will total more than one hundred units by the end of this year. Twenty-six of these will be in Europe. It is because of the many important installations in this part of the world that our company is making a great effort to provide the same service in Europe that it provides for customers in the United States. To this end a centre is being established in Switzerland as ARL-Europe. Many of you are familiar with the ARL Production Control Quantometer which has been produced for the last three years. A typical instrument of this type is shown in Fig. 1. The ARL High Precision Source, at the right, which contains a co&rolled high voltage spark unit and a Multisource unit, has proved its versatility time and time again during the methods development programme which has been very extensive over the last five years. At no time have we ever been at a loss for an adequate source to provide the utmost in reproducibility and sensitivity which can be obtained by spectrochemical methods. At. the centre is the grating spectrometer which is a vertical instrument usually fitted with close to the maximum allowable number of phototubes, thirty-five. At the left is the recording console providing Megrators for each of the phototubes, a single high voltage supply for all multipliers. an electrometer-type amplifier with supply. a strip chart recorder. and zero and sensitivity controls for each use of each multiplier. As may be seen. these controls provide an appearance of complexity to the Quantometer. However. this is only due to the fact that most typical instrument,s are constructed to handle the analysis of a large number of elements in two or three different base materials. Typical examples of this are unit,s to analyse low alloy steels, high alloy steels. and slags all in one instrument; likewise steels, copper base, and aluminium base alloys; also aluminium, zinc, and lead base alloys; and so on in various combinations. An alloy selector switch on these instruments allows the analysis of any of these base materials at a moment’s notice, and all with a minimum amount of calibration and readjustment. Just how t,his is accomplished will be discussed subsequently. Any careful study of direct-reading systems will demonstrate that the one adapted for the Production Cont,rol Quantometer provides the maximum in accuracy. speed. and simplicity from an operational sense. Integration of light from all spectrum lines is done simultaneously so that, any number of elements up to twenty can provide an integrated analysis for the whole sparking or arcing cycle, be this twenty or thirty seconds. It is t,his feature which provides the very 69

M. F. HASLER

high precision attainable with the Quantometer. The read-out system furnishes a pen and ink recording in duplicate requiring 1.6 second per recording, Thus, with five second prespark, twenty second sparking and integration, and thirty-two second read-out for a total of fifty-seven seconds, nineteen elements can be analysed. This is at the rate of one element every three seconds. This demonstrates the high speed of operation of the Quantometer. Finally, the repeated use of a single high quality amplifier and recorder makes for relatively straightforward servicing and maintenance. Having arrived at this basic design four or five years ago and having produced instruments of this type for several years, a good portion of our company’s development effort has recently been utilized to minimize servicing and maintenance. Three main trouble spots became evident, especially in twenty-four hours a day, seven days a week operation. Fit, the integrator relay contacts often failed to make adequate electrical connections. Though special cleaning tools were obtained, this meant that regular maintenance was required to assure setisfactory analytical service. The 8nswer to this problem is sealed integrators; by keeping dust and dirt away from the relay contacts, by making these co&a&s of palladium, and by providing two contacts in per8he1, integrator maintenance has been reduced to a very occasional thing. Second, the rotary switches which successively connect the various integrators and zero end sensitivity controls to the amplifier often wore out by the simple process of cutting themselves to pieces. Though they could be readily replaced by means of large Cannon connectors, this still occasioned maintenance which could bs minimized. Fig. 2 shows a new type of gold-plated rotery switch which not only takes the pl8ce of two of the older type, but provides at least three times the service. Thiid, the DC bridge amplifier, though it provided automatic compensation for zero drift, good linearity, and reasoneble stability, still left much to be desired in warm-up time and replacement tubs selsction. The solution to this problem wae to develop 8 unique vibrating condenser-electrometer type amplifier which draws no meaeuring current from the integrators, h8s practically no zero or sensitivity drift, is perfectly lme8r. and reaches full stability within one minute after being powered. Its operating principles am shown in Fig. 3. Essentially, it is a null typs instrument which balances itself electronically in about three cycle8 of the AC power supply. It has all of the virtues of 8 null instrument, providing an output voltage at low impedance which at all times is exactly equal and opposite to the ultra-high impedance input sign81 voltage. This 8ssures that there c8n be no drift in gain or hnearity. This output voltage appears Bcross the three resistors entitled sensitivity control, sum corrector, and zero control. By changing the ratio of the resistance across the recorder and the total resistance, by means of the sensitivity control, the full scale deflection of the recorder c8n be made any desired portion of this voltage. It is by switching in various sensitivity controls and sero controls that the voltage across each integrator can be read on 8 scale, appropriately espended 8nd adjusted to fit the direct-reading concentration scale provided for the auelysis of each element. In connection with this direct-resding feature, significant improvements have been made in the mechanism for inserting, holding and aligning the multiplecopy, direct-reeding analytical reports. Every I.6 seconds the recorder steps to the nest printed anaIytica1 scale and records the analysis of 8 part&l& element as a vertical line across the scale. The ball-pomt pen allows the simultaneous production of thrw analytical reports, one origin81 and two carbon copies. Another edvantage of the new amplifier is that it m8y be used as an electronic computer, in connection with multicomponent system analysis. By means of it, all intensity readings can be readily multiplied by a constant factor. Thii is accomplished by vsrying the fraction of t,he output voltage which the recorder meaeures. The variable resistor, termed the sum corrector, in Fig. 3 accompli&es this. The advantage of this in stainless steel analyeie, for instance, can be seen by referring to Fig. 4. Theoretical work in our laboratories has shown that in a multicomponent system the best representation for 8nal_vtical curves is, for example

Thai states that by plotting the concentretion of chromium against the intensity ratio of a chromium line to an iron line times 8 function of the concentration of iron, the internal standard, 8n invariant relationship will be obtained. By calibrating the sensitivity control of the iron internal standard in terms of f(Cp,) direct-reading analyses can be secured, if the iron content is known. Even in cases where the iron content is only known approximately, this approximate value can be used and 811other elements analysed. By summmg the iron value plus all determined elements, a total is ohtained which may be compared to lOOy$. If it deviates from lOO%, then by moving the sum corrector or sum reread dial to the sum value, the correct analyses may be recorded on another analytical report. This con all be accomplished

Fig.

1.

Production

Control Quantameter

with source on right, spectrometer

recording console on left.

Fig. 2.

Improved

Rotary

Switch.

in centre and

in

Quantometry

1952

without resparking the sample, se the original charges on the integrating condensers are used for both the preliminary 8nd tin81 analysis. This is possible only beceuse the amplifier removes no charges from the condensers to perform the reedings, and the system in general has such low leakage that no appreciable differences in readings are observed even after five to ten minutes. In Fig. 4, the original analysis is shown on tvpic8l direct-reading scdes ss the dotted line, the fin81 analysis as the solid line. The important point $. of course, that the direct-reading feeture of the Quantometer has been thus extended to complex alloy systems. ELECTROMETER

Fig. 3.

AYPLICIER

SchemEtic diagram of electmmeter

ampInier.

bother important Bdvance which shows considerable promise is a system of lamp celibration to minirnise the use of strmdard semples for instrument adjustment. One of the very importent features of the pmmnt Quantometers is the ease of calibration by the use of standard samples. By storing ohmgee on two sets of integrator condensers, one set with charges developed from the analysis of the st.8nd8n-lof high concentretions. 8nd the other with charges developed from a standerd of low concentrations, and bv providing 8 zero and sensitivity control for each analytical scale, such scales may be brought into ex&t coincidence with the standards at both the low 8nd high end of each. Such a method of standa&x8tion, though it only takes two or three minutes to perform, still leaves something to be desired in the wav of speed in 8 high production laboratory. The main variable in any direct-resding instrument employing multiplier phototubes is the sensitivity of these tubes. Such tubes are subject to fatigue and 8 vesietv of peculiarities which result in a drift m sensitivity with time. By pronely fatiguing these tubes, this c8n be minimized. However, even after all precautions are taken, it is found sdvanta@ous to check the c&bration of the instrument once every hour. The idea behind the calibration lamp is that if a lamp of constent intensity and constant spectral distribution were available, it could be utilized to &just the sensitivity of the photomultipliers SOthat each would be maintained at a predetermined value over long periods of time. Unfortunately no such lsmp exists. However, since all measurements are relative and the spectral response of various multipliers is not very different from one another, use of an ordinary tungsten fil8ment lamp turns out to be quite effective. One of the main duliculties encountered was to obtain a uniform field of illumination for 8ll multipliers which would be relatively independent of the particular lamp employed and its exact position. This ~8s finally achieved by developmg an mtegmting sphere. with a 6 x 50 mm opening, which is made to appear adlacent to the 35 x 50 mm rulings on the gratings. By having two lamps in the sphere, centered one on each side of the two inch length of the opening, either one m8y be used ss 8 calibrating source. By usmg,two flhunents in each lamp. this provides a total of four filaments ae protection against any one burning out at 8n inopportune time. By adjusting the voltage applied to each filament, the

71

X

F. HASLER

absolute intensities of the four filaments, at lea& as measured by the multipliers, can be made very similar. Differences in colour temperature between filaments, which must be present under conditions of equal absolute intensity, make for differences in calibration of any multiplier of less than one percent from one Clement to the next. However, the accumcy of measurement is good to one tenth of a percent, so that it is well worth while to calibrate each tilament against the others.

SOY REREAD

Fig. 4.

INTERNAL

STANDARD

CONGLNtRAtlON

Basic features of direct recording in a multi-component

system.

Having obtained a suitable source, the next problem is to make the calibrating light. se viewed by each multiplier, similar in intensity to the spectrum line being measured by that multiplier. Because of the great disparity between the intensities of the various spectrum lines employed. this is somewhat of a problem. However. by employing a limiting aperture along the length of each recetver slit which may be moved in and out of position in front of the slit and by masking either the spectrum line or the calibrating lamp, up to a hundredfold range of line intensities can be matched with a single calibrating lamp. A series of such length-limiting apertures, is mounted on a bar which supports them in front of the receiver slits and a small motor at the end of t,he bar moves the apertures in and out of position as required. To insure stability for the whole system, the sphere is water-cooled and the lamp is operated from a very accurate voltage regulator good to O-l?!, variation. A manually operated shutter allows the lamp to be turned on several minutes before use to assure equilibrium of the whole system. To minimise fatigue of the multipliers by the calibrating lamp, the shutter is opened just before calibration. This precaution is necessitated by the fact that continuous radiation. ptovtding the same average current in the multiplier as intermittent radiation, makes for much greater fatigue than high intensity, short duration, repetitive illumination. Table 1 shows some typical stainless steel analyses using the calibration lamp to adjust the instrument from time to time. Run five, after a twelve hour mterval since the previous analyses. shows the largest deviation in chromium values of any experienced. Thev are on the average low by 1.3”& of the quantity measured. Though this may appear very small, it still is an appreciable increase in error compared to 0.6?& which can be achieved with the longer procedure of utilizing duplicate sparkings of standard samples for calibration. However, the over-all error for the entire set of chromium values is O.S?/, of the quantity present. which certainly is approaching what oan be done with calibration standards. Thus, these results indicate clearly that the calibration lamp has some interesting possibilities which are well worth while investigating further.

Another important engineering development in the last few years is the design of Quantometers for multipurpose work. One of the prices paid for the high accuracy

Quantometry

in 1952

and speed of the Production Control Quantomet,er is an inflexibility of purpose. In the original concept of t’his instrument. it was designed to do a single job such as aluminum alloy analysis or low alloy steel analysis. However. as experience was gained. it became obvious that a multiplier and secondary slit on the 2516-4 silicon Table 1.

Direct-reading

(Calibration

Snalysis

of 18-S Stainless

by the Lamp Calibration

SteeL

Unit)

Sample

302

275

314

4319

298

Ni MI-I

18.87 8.32 0.77

17.50 i.94 0.80

16.92 9.04 o-43

18.54 12.52 1.94

20.06 9.60 1.27

Sum ber

Cr Chemicd

dnalyaes

Spectrochemical Date

_4nalyses* Tim

lOjl5!51

1.10 p.m.

Cr Si Mn

18.89 8.50 0.80

17.53 5.87 0.81

16.98 9.14 O-42

18.80 12.50 1.96

20.06 9.55 l-25

10/15/51

2.40 p.m.

Cr Si MI-l

18.89 8.65 0.79

li.58 8.00 0.80

Ii.27 9.17 0.42

18.82 12.80 1.94

20.00 9.80 1.24

10/15$1

8.20 p.m.

Cr Si Mn

18.67 8.40 0.81

17.47 7.98 0.81

17.2i 9.10 0.43

18.50 12.47 1.94

20.03 9.58 1.26

10~15i51

9.00 p.m.

Cr Si Ml-l

19.07 8.43 0180

17.53 8.05 0.78

17.40 9.10 0.41

18.90 12.72 1.93

20.36 9-58 1.26

lO’lSj51

9.00 a.m.

Cr Si Mn

18.58 8.29 0.80

17.18 7.81 0.78

16.56 8.96 0.41

18.84 12.21 1.94

19.15 9.42 1.23

10,‘16/‘51

9.55 a.m.

Cr Si Mn

18.68 8.20 0.77

15.43 7.58 0.79

Ii.07 8.87 o-41

18.83 12.10 1.92

20.00 9.35 1.22

* Average of d’uplicate analyses.

line, for instance. could be used for the accurat,e analysis of silicon in a wide variety of alloys, such as aluminum, magnesium. low alloy steel. high alloy steel, slags. etc. Thus. if proper swit,ching could be provided which would connect t,he silicon integrator to the amplifier at the proper position in the element reading sequence and at the same time would connect a separate zero and sensitivit,y control to the amplifier to allow calibration of an appropriate silicon scale, t,hen obviously the same spectrum line could be used to analyse a variety of base materials. To insure that’ each type of material can be analysed at, a moment’s notice without recalibration it is essential that separate zero and sensitivity controls be provided for each use of each spectrum line. In this way, once the alloy select,or switch is moved to the 73

M.

F. HASLER

proper position, the instrument is immediately ready to handle the analysis of the indicated alloy. This, as was indicated at the beginning of the paper5 is one reason for the apparent complexity of many of the latest instruments. An interesting example of this type of instrument is one designed to be of particular value to steel producers. The specifications are shown in Table 2. Besides analysing stainless steels for chromium and nickel in the high percentage or “A” range, silicon, manganese, molybdenum, columbium, titanium, copper, and so on can be readily determined in the one-tenth to a few percent range at the same time, with a spark-like Multisource discharge. The “ B ” range provides an analysis of residuals such as low molybdenum, tin, aluminum, vanadium, lead, and copper by means of an arc-like Multisource discharge. Tantalum, as an alloying element, can also be determined with this discharge. By switching to the “ C ” range of the instrument, a low alloy steel sample can be analysed immediately for most alloying constituents. The “ D ” range provides residual analyses for this type of steel and boron analyses in the alloying range from 0.0005-0~05 y/& The “ E ” range is used to check and adjust the position of the receiver slits with respect to the spectrum lines by means of the monitor lines and slits provided. This is used just before recalibration of the analytical scales. The “ F ” range provides for the analysis of steel making slags. Types used in stainless steel production as well as low alloy steel making can be readily analysed with the Quantometer when set to this range. On studying the specifications of this instrument it is apparent that many spectrum lines are used repeatedly in the analysis of various basic types of samples. By having direct-reading analytical forms for each range, analyses can be performed on a speed basis in any and all ranges as required. This provides an introduction to one of the latest and most important developments of the Methods Laboratory of the Quantometer Division of ARL-slag analysis. The method was first developed in connection with steel-making slags. It has been extended to the accurate analysis of cements, zinc smelter slags, copper smelter concentrates and slags, and so on. It appears to make possible, in t,he field of non-metallics, the same extension into high percentage analysis with the Quantometer as has occurred in the metals field. The method is based on a rapid electric furnace fusion; 1 g of lithium carbonate and 1 g of boric oxide are mixed with 0.5 g of pulverized slag which will pass a 100 mesh screen. These two added fluxes may be weighed and mixed previously to analysis to speed the operation. This mixture is poured into a graphite crucible made by drilling a 18 mm hole, 15 mm deep into a graphite rod 26 mm in diameter and 18 mm in length. This is heated in an electric furnace at 1000’ C, for five minutes. The graphite crucible is removed and cooled, and the glass-like bead which results is easily pried from the bottom. The bead is crushed in a Plattner mortar with a half dozen hammer blows. The coarse particles are sieved out with a 100 mesh screen A portion of the fines are ground further and passed through a 325 mesh screen. A half gram of this fine material is weighed up roughly and added to an equal amount of briquetting graphite. These are mixed together and pressed into a 12 mm diameter briquet, at 80,000 pounds per square inch. Control of the particle size of the fused material is a very important factor in determining both the accuracy and precision of the analyses. 74

2

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I __ I

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Totals:

2.0+

Integrators I’hototulm Chennela SeloctQl~

O.I-

0.24.0 3.0~16~0

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3l. F. HASLER

This entire process of sample preparation takes from t,en to fifteen minutes, depending upon the equipment available and the speed of the operator. The graphite crucibles are made of ordinary grade graphite rod and may be cleaned for re-use by touching them up with the 18 mm drill employed t.o produce them. For the Quantometer analysis, the briquet is clamped in a flat-plate briquet holder. About 0.7 mm of flat surface is removed Fith a razor blade to provide conditions identical with those used for subsequent analysis. Analyses are made with a three second prespark and a twenty second sparking, utilizing the Nultisource unit. Two microfarads, fifty microhenries, and residual resistance are employed in the power circuit with the igniter at maximum power. The sample is the positive electrode.

L

I/

lo

1

20

I

I

I

t

x, 40-WA &LYl6o x, * L

Fig. 5.

Analytical

ewe

1

q” 00

for silica in slags.

By utilizing the shaving technique, on the briquet, duplicate or triplicate analyses can be made in rapid succession. Also, very long briquets of standard samples can be used over and over again for calibration. While on the subject of standard samples, it should be pointsd out that because of the good fluxing obtained, standard samples may be synthesized from oxides and carbonates, and reduced to a common physical and chemical form, just as production samples are, by the furnace treatment. The real test of any method, is of course, what it will do in the analysis of a wide variety of slags obtained from many sources. The following group of figures, showing analytical curves for the various elements or compounds of interest. shows clearly what this method has accomplished. It should be pointed out that the chemical analyses were, in general. routine analyses as provided by the steel producers. The results speak very well for the high quality and uniformity of chemical slag analysis as practiced both in the United States and in Europe. The first working curve, Fig. 5, is for Si02, determined, of course, as silicon by the spectrochemical met,hod. As may be seen from the abscissa, the added lithium is used as an internal standard. This allows direct-reading determinations of all constituents and eliminates all calculations. The first feature of interest is the 76

Quantometq

in 1952

effect of iron on the silicon determination. Fortunately, most slags divide themselves into iron cont,ents of less than 3 0; or more than lOO,&. Two working curves The effect of high iron appears as thus suffice to cover these two main groupings. an increase in spectral background causing a parallel shift of the analytical curve. Increased Cr,O, content in the slag has the same effect so that the SiO, determinat,ion must be corrected for it. The results obtained in trying to eliminate the fusion step are clearly indicated Even for slags of the same type, serious by the block square determinations. errors result if analysis is attempted by the addition of the lithium carbonate and boric oxide, but without fusion. In this connection it should be pointed out, in all fairness, that ot,her discharge conditions could be utilized to minimise these

Y)2Q30405060x)809000 +$# Fig. 6.

Analytical

curve for iron in slags.

However, such discharges are quite inferior differences in the unfused samples. in precision to the one utilised so that one loses both in accuracy and precision by employing such methods. &4similar analytical curve is obtained for CaO, determined as calcium. Here iron and chromium have qualitatively the same effect on the curve as they had in the case of the SiOz det,erminat,ion. This explains why certain relatively simple spect,ro-chemical methods for determining the calcium-silicon ratio have enjoyed a moderate success--the effect of variable iron content, tends to cancel out to some ,extent. However. this is only fortuitous and thus only partially the case. Another element of great interest is iron. This is present as Fe0 and Fe,O, in the slag, the amounts of each oxide depending upon the calcium-silicon ratios. Aft,er fusion. a wide range of iron values may be fitted to a single anal_ytical curve, as shown in Fig. 6. However, unfused slags provide very erratic values, presumably due to the particular forms of oxide present in each sample of slag. A similar simple working curve is obtained for manganese wit,h no apparent matrix effects. Chromic oxide. so important, in the production of stainless steel, can be readily determined in slags. A simple linear analytical curve is obtained with no visible mat,rix effects.

iM. F. HAXE.E

Table 3 shows what can be expected in the way of reproducibility. For a single sparking the results are not on a par with good metal analysis in which less than Table 3.

Prwihm

of Slag Analysis

-

c-Element

OT

SiO, Z? ivfgo TiO, cr203

Fe Mll P

TY+=~ Adpia

Standmd Deviation For Single Analysis

1940 4.93 34.5 7.14 o-54 14-64 21.10 4.20 1.74

All numbers

* f * * * * i * f

0.46 o-10 o-41 o-11 o-017 0.34 0.27 o-11 o-17

Coejicient

of VczTtilm

2.3 2-l 1.2 1.6 3-2 2.3 l-3 2-6 10.0

in percentage.

one percent precision may be obtained on many elements. However, by the use of duplicate sparking, and a third, if good agreement is not achieved, escellent results can be readily attained. Due to the use of fused samples, the accuracy of analysis closely approximates the precision which is something which cannot be said for methods which do not employ fusion. In closing, it is worth while reviewing some of the highlights achieved in metal analysis in our methods laboratory and in field installations. The results shown in Table 4 are noteworthy, as they indicate the penetration of Quantometric analysis into more and more fields of metal analysis. 51ost of these fields were closed, at least in the higher percentage ranges, to spectrographic analysis because of the inherent errors introduced by the recording device, the photographic emulsion. Where these errors are of the order of magnitude of one percent for photographic measurements, they are of the order of O-1o/ofor multiplier phototube measurements as obtained with the Quantometer. Thus, the most serious limitation to accurate analyses by direct-reading methods today is the sample, rather than the instrument. For instance, the analysis of zinc in the 30-400,; range in brasses provided errors of from O-5-1 % of the quantity measured, for laboratory produced standards. This error dropped to 0.3-0.40,b as soon as routine production samples were obtained from large, well-stirred furnaces. ,4nd so it goes, routine production samples taken in wkll-managed plants almost invariably provide lower deviations than specially prepared small batch samples. Thus, the real challenge today in spectrochemistry is the development of methods of sample preparation which will allow the full use of the accuracy of direct-reading instruments. Perhaps the British Ken-Ferrous Research Association and British Cast Iron Research Association will meet the challenge with the aid of their direct-reading instruments in the years to come. 78

Qusntometry

Tabb 4.

Alurninium

Stainless

Alloys

Steels

in 1952 _4nuJyaia Highlights

Quantam.eter Xetd

Si

=

12.2Sqd

cu

=

8.210

Cr

=

Xi

=

cr xi

= ZI

,O

ls+2~o 8.217&

Mn =

1.3553; 3.229; 0.32:/,

zinc Alloys

Al &

= =

4.05?& z.s;c$;

Lead Alloys

Sb

=

copper

Xi Pb Zn

= 31.77% = 1.82% = 35.82:!

Low Alloy

steels

Alloys

12.11%

Deviations experienced for single analyses of samplea of high uniformity, as compared to average of twenty determinations.

79