593
Anal. Chem. 1980,52, 593-594
Mechanical Apparatus for the Correction of Zero Fluctuation Effects on Automatic Chart Recordings Michael G. Burnett Department of Chemistry, Queen’s University, Belfast BT9 5AG, Northern Ireland
Consecutive spectroscopic scans are frequently used t o detect products or mechanistic features in chemical studies. Ideally the apparatus used should record t h e spectrum with respect t o a n unvarying zero base line but, in practice, a drifting base line or a variable instrument distortion is often added t o t h e required signal. It is possible to obtain instruments which will present perfect spectra b u t generally routine equipment does not a t present give such a performance. T h e imperfect spectra m u s t therefore be corrected so t h a t the records are directly comparable. Usually this is done by transcribing t h e record into numerical form a n d t h e n correcting for the fluctuating zero on a point by point basis either by hand or computer. This method is so time-consuming that it is rarely used unless t h e result is of special importance. However, a cheap a n d reliable mechanical method can be used which reduces the time needed for full correction of chart records to a m a t t e r of minutes a n d has t h e subsidiary advantage of allowing scans recorded on separate sheets t o be superimposed in a single figure. I t is based on t h e principle of t h e carpenter’s profile gauge. T h e microprocessor equipment required for this work is currently a thousand times more expensive.
strengthening spine
r
(a)
_ _ I -
///////’//////I foam r J b b e r
-
EXPERIMENTAL Apparatus. The apparatus described in this account was devised to treat results recorded using a Unicam SP8000 spectrometer but the principle is applicable t o all chart recording instruments. As shown in Figure 1, a plywood and batten frame
’
rubber
feet
3..
,
+
--’v’
--+
Figure 1. Sketch plan of apparatus for the correction of automatic chart recordings
d
Figure 2. Diagrammatic illustration of the method. (a) Original curve and zero (- - -) with required corrected zero (+-I marked on rods. (b) Zero marks on rods (t) matched to observed zero ( - - - ) and curve registered on rods (+). (c) Rods realigned to edges of the box. (d) Corrected curve traced from the final position of the marks made in Stage b above 0003-2700/80/0352-0593$0 1.OO/O
0 1980
American Chemical Society
Anal. Chem. 1980, 52, 594-595
594
is constructed with 2 removable sides (a) and (b), held in place by heavy clips. The two fixed sides are raised by a 0 5 m m edging so that the original record sheet can be slipped freely under the sides and aligned with its lower edge parallel to the bottom of the frame. I t is held in position by the lower removable batten (b) and the frame is filled with 80 glass rods (3-mm diameter) cut so as to be ca. 1 mm longer than the frame. These are held in place by clipping on the upper batten (a) which is lined with a 1-cm thickness of foam rubber. The upper ends of the rods are securely held by being embedded in the rubber. Method. Sheets of tracing cloth or transparent plastic are prepared for the final corrected spectra. They are cut to fit in the rectangular opening defined by the sides of the frame and the calibration lines are traced onto them before use. The chart carrying the spectrum is now clipped in place with the rods laid over it. The zero line of the chart is marked on the rods with a spirit-based fiber-tipped pen (Staedtler "Lumocolor". Stage a, Figure 2) after which the removable sides are unclipped. A heavy glass rod cut to fit the breadth of the tray is useful for gently easing the inked zero line so that it exactly falls on the recorded spectral blank scan. The rods are then clamped in this position using the upper side (a) of the frame, (Stage b. Figure 2). The batten (a) had to be strengthened as shown in order to provide an even pressure a t the center as well as the edges of the tray. The spectral scan required is now marked on the rods (Stage
b, Figure 2). This is made easy by the focusing effect of the round glass rods which breaks the scan into a series of almost straight sections. The spectrum is effectively divided optically into a record using ca. 80 channels. The transcription of, for instance, sharper spectra would require smaller diameter rods or the use of an expanded wavelength record. If a composite figure is required, second and further charts are slipped under the chart originally in the frame before the latter is withdrawn from the bottom. Unless this is done it is difficult to get the new chart in place without disordering the glass rods in the tray. After aligning the new chart with the lower edge of the frame and adjusting the rods to match the zero scan as before. the new scan may be marked on the rods. Varying ink colors are used if any possibility of confusion should arise in this or subsequent scan transcriptions. Finally, the frame is reassembled and the rods returned to their initial arrangement, (Stage c, Figure 2). This action mechanically corrects all the observations to a common true zero. The transparent tracing cloth or plastic sheet can now be dropped into the tray over the rods and the spectra copied (Stage d , Figure 2). The ink marks on the rods readily dissolve if they are stood vertically in a tall beaker containing methyl alcohol and they require only drying before re-use.
RECEIVED for retiew July 24, 1979. Accepted October 26, 1979.
Modified La Mer-Generator for the Production of Small Amounts of Sulfuric Acid Aerosol Reinhard Niessner and Dieter Klockow Department of Chemistry, University of Dortmund, P.O. Box 50 05 00, 0-4600 Dortmund 50, German Federal Republic
Y i t h t h e growing interest in t h e determination of atmospheric sulfuric acid ( I , 2), there is also a need for a reliable a n d easily operatable sulfuric acid aerosol generator. Sulfuric acid aerosol generation methods which are currently used include t h e atomizer-burner combination as described by T h o m a s et al. ( 3 ) ,t h e Berglund-Liu generator ( 4 ) ,t h e conversion of sulfur dioxide t o sulfur trioxide ( 5 ) ,or t h e reaction between SO3 and H 2 0 (6, 7 ) . T h e above mentioned aerosol generation methods possess one or more of t h e following disadvantages. (1) T h e emission of only particles which are chemically identical is not necessarily assured. (2) T h e aerosol particles might n o t h e balanced with respect t o electrical charge. (3) Stability in t h e low-output mode is not satisfactory. (4) Difficulties are encountered in generating H2S04droplets of less than O.l-pm diameter. ( 5 ) Construction of these devices may b e expensive. T o avoid such disadvantages, it was preferable t o use a condensation type generator as described by La Mer e t al. ( 8 ) o r by S p u r n 9 and H a m p l (9). T h i s generation method possesses t h e following advantages. (1) Only chemically identical particles are emitted. (2) A homogeneous aerosol with a narrow size range can be produced. (3) Droplets with diameters less t h a n 0.1 pm carrying a maximum of one unit charge (10) can be generated. (4) T h e generator can easily be constructed a n d adjusted to different 0003-2700/80/0352-059450 1 OO/O
Table I. Working Parameters of the Modified La Mer-Generator temp. of
boiler temp.
heating section
QbollerQ
353-373 K
421 K
80 Lih
'Q
QCilla
,40
L/h
surracc of pool
- 1 0 cm2
flow rate.
modes of operation with respect to o u t p u t a n d size range. For our investigations of artifacts in filter sampling of sulfuric acid aerosol ( 1 1 ) t h e L a Mer-generator in its original construction could not be used because of its high acid output. TVe have modified t h e generator with t h e practical considerations of simplicity, low output, and output stability in mind. T h e construction details are shown in Figure 1. A pre-thermostated nitrogen stream flows over t h e surface of a thermostated sulfuric acid pool of t h e same temperature. T o obtain a n aerosol with low humidity, 99% sulfuric acid is used. I n addition t h e nitrogen is pre-dried by flowing through silica gel. A small amount of sulfuric acid vapor is swept into t h e nitrogen stream, breaks down to molecules in t h e heating section (421 K ) a n d t h e n h a s t h e possibility t o recondense t o small droplets in t h e following cooling section. T h e result is homogeneously grown particles. T h e emitted C 1980 American Chemical Society
