Some characteristics of an intensified photodiode array spectrometer system for use in plasma emission spectrometry

Some characteristics of an intensified photodiode array spectrometer system for use in plasma emission spectrometry MASAAKI KUBOTA, Y~SHIMASA FUJISHIRO and RYOHEI ISHII)A National Chemical Laboratory for Industry, Higashi, Yatabe. Ibaragi-ken, Japan 305

(Received I I January

1982; in revised form 9 April 1982)

Abstract-A IOU-element silicon photodiode array with a microchannel plate image intensifier was coupled to a Czerny-Turner spectrometer and basic characteristics of the spectrometer system were studied using an Hg lamp, a hollow cathode lamp and a d.c. argon plasma as emission sources. The intensifier proved to be useful to enhance signals without increasing the electronic background. The signal to dark current ratio of the cooled photodiode array was larger than that of a photomultiplier (HTV R457) in the wavelength region above 380 nm. Effects of entiance slit width, integration time per scan and intensifier gain on signal to background ratios and signal to noise ratio are presented and optimum measurements using the system are described.

conditions for emission

1. INTRODUCTION last decade numerous research papers have appeared on the possibility of using linear silicon photodiode arrays for spectrochemical analysis. Some of the papers [l-g] describe the basic electronic and optical characteristics of the diode arrays and their use as detection devices for spectral information. In atomic emission spectrometry, photodiode array spectrometers have been utilized for the identification of flame [3,4], direct current arc [9, IO] and inductively coupled plasma (ICP) Ill, 121 emission spectra, the measurement of signal to noise ratio characteristics of an ICP 1131, and the determination of copper in aluminium alloys using medium voltage spark excitation [S] and constituents in pressed powder pellets by means of a Grimm glow discharge lamp [6,14]. Also, in recent reports, diode array detection has been employed as an effective means to measure spatial profiles of emission from spectral sources such as flames and ICPs [lS-201. The main limitations of diode array detection systems for spectrochemical applications are lower sensitivity, inferior spatial resolution and narrower dynamic range IN THE

[l] P. W. J. M. BOUMANSand G. BROUWER.Spectrochim. Acta 27B. 247 (1972). [2] P. W. J. M. BOUMANS,R. F. RUMPHORST,L. WILLEMSENand F. J. DE BOER, Spectrochim. 227 (1973). [3] G. HORLICKand E. G. CODDING,Anal. Chem 45, 1490 (1973). [4] G. HORLICK,Appl. Spectrosc. 38, 113 (1976). [S] H. BUBERT,W.-D. HAGENAHand K. LAQUA, Spectrochim. Acta 338,701 (1978). [6] H. BUBERT,W.-D. HAGENAHand K. LAQUA, Spectrochim. Actn 348, 19 (1979). [7] H. BUBERT,W.-D. HAGENAHand D. STOWER,Spectrochim. Acta 348.289 (1979). [8] H. KAWAGUCHI,K. OTA, T. ITO and A. MtzurKE, J. Spectrosc. Sot. Japan 29, II5 (1980). [9] E. G. CODDINGand G. HORLICK, Appl Spectrosc. 27, 366 (1973). [IO] G. HORLICK,E. G. CODDINGand S. T. LEUNG, Appl. Spectrosc. 29,48 (1975). [II] K. R. BETTYand G. HORLICK.AppL Spectrosc. 32,3l (1978). [I21 S. K. HUGHES, R. hf. BROWN,JR. and R. C. FRY, Appl. Spectrosc. 35, 3% (1981). [I31 E. D. SALIN and G. HORLICK,Anal. Chem. 52, 1578 (1980). [I41 H. BUBERTand W.-D. HAGENAH,Spectrochim. Acta 36B, 503 (1981). [IS] M. FRANKLIN,C. BABERand S. R. KOIRTYOHANN,Spectrochim. Acta 31B, 589 (1976). [I61 T. E. EDMONSand G. HORLICK,Appl. Spectrosc. 31,536 (1977). [I71 H. KAWACUCHI,T. ITO, K. OTA and A. MIZUIKE. Spectrochim. Acta 3SB. I99 (1980). [I81 G. HORUCK and M. W. BLADES,Appf. Spectrosc. 34, 229 (1980). ]I91 M. W. BL.AIXSand G. HORLICK,Appl. Spectrosc. 34.6% (1980). [201 M. W. BLADES and G. HORUCK, Specfrochim. Actn 36B, 861 (1981). 849

Acta 28B,

M. KL’IIIII 1. Y. Fi~lI\III~cl ;1n,l K. I\ill[,\

850 wherl

compared

systems

capability analysis.

to conventional

of We

phofoniulliplicr-exit

4it systems. Even so these array

bccausc the multichannel nature and the signal integrating the arrays are particul:u-ly suited to multielement spectrochemical have coupled an intensified photodiode array to a Czerny-Turner

nre still

attractive.

spectrometer as a detector for plasma emission spectrometry. In this paper, the basic characteristics of the photodiode array spectrometer system and the optimum conditions for emission measurements using the system will be described. 2. EXPERIMENTAL 2. I Apparatus

and procedure

A Tracer Northern TN-1710 IDARSS intensified photodiode (PD) array system was used. The system includes an intensified PD array (TN-1223-41) with a high voltage intensifier controller, a signal processer (TN-1710-21) which consists of a signal amplifier and a 12-bit A-D converter, and a microcomputer based signal analyzer (TN-1710-2K) having a 2K words of data memory and a CRT display screen. The intensified PD array has a microchannel plate image intensifier that is fiber-optically coupled to a thermoelectrically cooled PD array. The intensifier gain can be varied from 38 (minimum) to 8933 (maximum) by changing the setting of a variable resistor which varies the voltage supplied to the intensifier. The dimension of a single sensitive element of the PD array is 12.7 pm x 2.5 mm. The PD array is composed of 1024 elements and interspaces between the elements. Each interspace has a width of 12.7 pm. The array detector was mounted in the exit focal plane of a JEOL 1.25-m spectrometer (model JSG-IZSB, 1200 rulings mm-’ grating, blazed for 300 nm). The reciprocal linear dispersion of the spectrometer is 0.62 nm/mm and the array has a total width of about 26 mm; thus the spectral range covered simultaneously is approximately 16nm. Spectra displayed on the CRT were printed out on a strip chart recorder (Hitachi, model 056) when needed. For comparison, a photomultiplier measurement system was also employed. A Hamamatsu TV R457 photomultiplier tube was used at a supplied voltage of 610 V and the output signal was amplified with a direct current picoammeter (Takeda Riken, model TR-8641). The exit slit of the spectrometer was adjusted to a width of 30 pm and a height of 2.5 mm. Thus the radiant flux received by the photomultiplier is approximately 2.4 times larger than that received by a single element of the PD array. Signals were determined by scanning a spectrum at a speed of 0.5 nm min-’ across the wavelength interval of the spectrum. The experimental arrangement is shown schematically in Fig. 1. An Hg pen-ray lamp, a Pb hollow cathode lamp and a d.c. capillary arc plasma (CAP) were used as emission sources. The CAP source used in combination with a sample introduction system for solutions has special

Fig. I. Schemzltic diagram of experimental set-up.

851

Characteristics of an intensified photodiode ;Irr;iY SpeCtr(~~llCtCrcY$tcm features

of stable emission,

and relatively

inexpensive

glass concentric

nebulizer,

For all measurements, plasma-forming 2.2 Reagents For plasma magnesium

economic

flow rate of plasn~

apparatus

[21-231.

a spray chamber

an argon stream

gas and the plasma

emission

(99.99%)

for the measurements

3.1 Spectral

introduction device

at a rate of

standard

A IO pg ml

’

consists

for producing

of a

dry aerosol.

of 5.2 A.

solutions

were prepared

magnesium

of

Power consumption system

I .I I min ’ a~ a ncbulizing and

at an arc current

magnesium

unless otherwise

3.

was employed

water.

gas. 10~ electric

sample

and a desolvation

was operated

measurements,

and deionized

The

solution

with pure

was employed

stated.

RESULTSAND DISCUSSION

half-width

PD array spectrometer system used in this study results in a wavelength coverage of 0.016nm per diode, this number does not represent the spectral resolution which ultimately can be attained. BOUMANS et al. [2] measured the spectral resolution quantitatively for the Hgdoublet 313.155-313.183 nm with photodiode array and photomultiplier-exit slit systems, and reported that both detector systems gave virtually the same spectral resolution. As the doublet could not be resolved with our PD array spectrometer system, we have measured the half-width of the Hg I 2%.728 nm line instead. A Hg lamp was used for the measurement. With a lO-pm wide entrance slit of the spectrometer, the half-width was 0.081 nm which was approximately three times wider than that obtained using the photomultiplier-exit slit system. The rather poor spectral resolution and the larger half-width observed with the PD array are probably due to blooming. It is likely that the microchannel plate image intensifier degrades the overall resolving power of the PD array detector. The effects of increasing entrance slit width and intensifier gain on the spectral half-width attained with the PD array system are illustrated in Fig. 2. With a fixed setting (minimum) of the intensifier gain, the half-width provides a minimum at a slit width of 15-20 pm and increases with increasing slit width. While, with a tied slit width (10 pm), the half-width increases gradually with increasing intensifier gain and becomes almost constant in the gain range above 2000. The increase in the half-width observed at low slit width seems to be caused by diffraction at the entrance slit, since similar trend was seen with the photomultiplier-exit slit system. Although

the

o.oJ-----20

0

40

60

Slit width,

Fig. 2. Half-width trometer

80

L-l----

100 30

was changed.

gain of the PD array.

(b) With

1000

10000

Intensifier gain

of Hg I 2%.728 nm line plotted

and intensifier

100

pm against

(a) With

a fixed slit width (10 pm),

entrance

slit width

a fixed intensifier intensifier

of the spec-

gain (38). slit width

gain was changed.

In plasma

emission

measurements,

net signal often

represents

the difference

be-

tween the signal of line plus background with a sample solution and that of background with a blank solution being nebulized at the same rate as the sample solution. However, the background level with a sample solution is not always equal to that with

a blank solution. So, as illustrated in Fig. 3, the plasma background signal was also measured in a spectrum of the sample solution as the average value (B,,) of background points at 212 channels from the peak (channel x) of an analysis line. Twelve channels correspond ‘0 about 0.18 nm of wavelength. Z,* expresses the net integrated signal between rt12 channels across the peak and Slz is the net signal obtained by subtracting the plasma background signal from the peak height (P). D represents the electronic background of which the major component is the array dark current. & is the background signal at the line wavelength (A) for a blank spectrum and S, is the signal obtained by subtracting B,, from P. Table 1 presents the relative standard deviation (RSD) for ten repetitive measurements of the Mg II 279.553 nm line intensity using S,, Slz and Z,, as the signal. It can be seen in Table 1 that use of the integrated signal (Z,,) yields poorer precision. This is because the dark current fluctuation and the plasma background fluctuation due to the instability of the arc plasma increase the RSD especially of signals on the foot of the spectrum, i.e. at channels near the background points. Hence S, and S,* were used as the signal in the plasma emission-PD array measurement studies described below. 3.3 Effects of slit width, integration time and intensifier gain on signal to background ratio Signal to background ratios (Mg II 279.553 nm) were determined under various measurement conditions of the PD array spectrometer system. Figure 4 shows the effects of entrance slit width on the peak height, background signals and signal to background ratios. The measurements were made using a 0.2-s integration time per scan, 10 averaged scans and minimum gain setting. The figure reveals that a linear response is attained for the peak intensity in the slit width range up to 20 pm. The ratio of S,, to BOgave a maximum at a 20-r*.rn slit width. This slit width is nearly equal SAMPLE SOLUTION ANALYTE

BLANK SOLUTION

PEAK

BACKGROUND DARK CURRENT

ZWe\EVEL

x-12 i-O.18

wavelength[nm] Fig. 3. Illustration

Table

I.

Relative

of symbols

standard

x x+12 a x+0.18 used in plasma emission-PD

deviation

(%) for signals S,,. SIT

x x array measurement

studies.

and Zlz of Mg II 279.553nm

1.76

I .99 I.Yb

line.

Ch;uwzteristics

of an intcncified

photc~dic~tlc m-ay

spcclromcter

system

853

x---x--I-x-

Mg

$

2 ”

U 279.553

“m

soo

20 Slit

Fig.

4.

30

40

width.

vm

50

Peak height, plasma background signals and signal to background ratios of Mg If 279.553 nm line plotted against entrance slit width of the spectrometer.

to the width of a sensitive element plus an interspace. The ratio of Sr2 to B12 showed a peak at a slit width smaller than the slit width at which the So/& reached the maximum. This is because the background points for Bu which lie on the foot of the spectrum becomes hi (i.e. Bu becomes larger than &) as the signal increases. Figure 5 shows changes in the peak height, plasma background, dark current and signal to background ratios with combination of integration time per scan (IT) and number of scans (NS). The total measurement time (IT x NS) was kept constant at 10 s. A 1 pg ml-’ of magnesium solution was used in this experiment. It is clear from Fig. 5 that the observed dark current ( = dark current per scan x NS) decreases drastically as the integration time increases from 0.01 to 0.5 s. Thus the best signal to dark current plus plasma background ratio was achieved with integration times longer than 0.5 s. If the plasma background level is much higher than the dark current level, the ratio would be independent of the integration time just like the signal to plasma background ratio. In such a case, shorter integration times may be used within a range that the dark current noise is not significant. The effects of the intensifier gain on the peak height, background signal, dark current and signal to background ratios are presented in Fig. 6. When the intensifier gain was raised from the minimum to the maximum, the peak height and the background signal changed over a range more than two orders of magnitude. Although an improvement in the signal to plasma background ratio cannot be achieved by increasing the intensifier gain, the intensifier enables one to enhance the signal and background without raising the dark current, yielding a relative reduction of the dark current level compared with the plasma background level. The signal integrating capability, which is an important property of PD arrays, is limited by the dynamic range of the A-D converter (4096:l). Within the range, the sensitivity of the array is linearly related to the integration time. However, using longer integration times increases the dark current per scan and causes a decrease of the linear range left for signal integration. In order to secure a desired linear range for

M.

x54

KUHOTA,

Y. FUJISHIM

and R. ISHII>A

0.05 -i

m

A 0.02 * v; 0.01

0.01 1000

Fig. 5.

0.05 0.1 0.5 1 Integration time. s 200100 20 10 Number of scans

5

10

2

1

Peak height, plasma background signal, dark current and signal to background ratios plotted against integration time per scan and number of scans.

Mg

30

Il279.553

100

nm

1000

10000

Intensifier gain Fig. 6. Peilk height.

plasma

background signal, dark current and signal to background plotted against intensifier gain.

ratios

Charackrisrics of an intensifiedphotodiode array spectronleterFYstenr

analyte

signals. it will be recommended

times and higher intensifier 3.4

of shorter integration

to use combinations

gains.

of slit width, irrtqyltiou

Effects

855

time

crrrcl irltertsijicr

gclirr OII siwtrl

to

Signal to noise ratios were determined for the Mg line emission. defined in terms of plasma background noise i.e. the standard deviation

noise rdio “Noise” ws based on ten

replicate measurements of B,_, values. The effect of entrance slit width on the signal to noise ratio is shown in Fig. 7(a). The figure indicates that the signal to noise ratio increases

with the increase of slit width,

can be concluded spectrometer

reaching

a maximum

from Figs. 2,4 and 7(a) that the optimum

at a JO-pm slit width.

slit width for our

It

PD array

is in the range of IO-30 pm.

of the signal to noise ratio on the integration time per scan and the number of scans is illustrated in Fig. 7(b). Obviously the shorter the integration time per scan the smaller the ratio. It is presumed that, with shorter integration times, the The dependence

array

dark

current

noise becomes

the dominant

noise

in the plasma

emission-PD

for ICP emission-PD array spectrometry by SALINand HoRLICK[13]. In spectrochemical applications, it will be desirable for our PD array system to select integration times in the range of about 0.2 to about 2 s so as to achieve higher signal to plasma background plus dark current ratios, higher signal to noise ratios and satisfactory dynamic range for signal integration. The effect of increasing intensifier gain on the signal to noise ratio is shown in Fig. 7(c). The figure reveals that the ratio is less sensitive to the intensifier gain. This is probably due to the fact that, for the d-c. argon plasma, source flicker noise is the major source of background fluctuations, where the noise (i.e. plasma background noise) as well as the signal increases with intensifier gain. It should be noted from the result that the intensifier would not give a remarkable effect on the improvement in the detection capability of plasma emission spectrometry. One major advantage of using higher intensifier gains will be the ability of enhancing signals without increasing dark current as indicated by the data presented in Fig. 6. array detection

system. Similar

observations

have been reported

3.5 Sensitivity Figure 8 compares emission spectra of a Pb hollow cathode lamp for a short (214.5-230.5 nm) and a long (356.0-371.5 nm) wavelength region with the photomultiplier-exit slit system and the PD array detection system. In the long wavelength region, similar peak intensities are observed for the spectra obtained with the two systems, though the PD array spectrum shows inferior resolution. In contrast, in the

10

20

30

40

50 0.01

0.1

Integration time, 5

Slit width, vm

1000

100

10

Intensifier gain 1

Number of scans Fig. 7. Signal to noise ratio as a function scan and number

of (a) entrance

of scans and (c) intensifier

slit width,

(b) integration

gain for Mg II 279.553

time per

nm line emission.

$1. KkNrcclr,\. Y.

PHOTOMULT

214.5

- 230.5

dntl

K.

I PL I E R

I\rtlr).\

PHOTODIODE

214.5

nm

- 230.5

ARRAY

nm

Intensifiergain: 2230

Full scale: IO pA

Fig. 8. Spectra

FllJl~lllhil

of a Pb hollow cathode lamp obtained with the photomultiplier-exit and the PD array system.

slit system

short wavelength region, the photomultiplier is clearly more sensitive and shows lower dark current noise compared to the PD array. The comparison of sensitivity is further illustrated in Fig. 9 in terms of the ratio of signal to dark current ratio of the PD array to that of the photomultiplier. Fifteen Pb lines in the region of 200420 nm were used for the signal measurements. The intensifier gain wis set to 2230. The curve in Fig. 9

,.,L-----300

200

Wavelenyth, FIJ.J.9. Ratio of silwalto dark current with

the photomultiplier

(PM)-exit measured

ratio obtained

400 nm

with the PI> array systerr. to that obtained

slit system as a function of wavelength. for t’h hollow c;lthotle lamp spectra.

Signals

were

Characteristics

of an intensified

that the intensified

demonstrates

only in the wavelength Generally

cooling

photodiode

PD array

array

spectrometer

is more sensitive

857

system

than the photomulliplier

region above 400 nm. of the array

is an effective

[4. 131. In case of the array detector

means

to reduce

the dark current

used in this study, the dark current

was reduced

by approximately SO% as the temperature of cooling water was decreased from 20 to 1°C. Thus the signal to dark current ratio for the cooled PD array excecdcd that for the photomultiplier in the wavelength region above 3X0 nm. 4. CONOAJSIONS The intensified electronic limited

PD array system enables enhancement

background

sensitivity

spectrochemical

of

which

consists

the array

applications.

mainly

of detector

is the greatest

of signals without dark current.

disadvantage

Photomultiplier-exit

slit systems

for

increasing

Even

so the

use in emission

should

be used to

measure weak signals, because, in the wavelength region below 300 nm, the signal to dark current ratio of a photomultiplier (HTV R457) was roughly a factor of IO larger than that of the intensified trations

are sufficiently

PD array.

high to apply

However,

for many situations,

the PD array

system

analyte

to quantitative

concenmeasure-

ments. It will be necessary to demonstrate the applicability of the system to practical analyses, for example, determinations of major and minor constituents in alloys and minerals.

Such a study is being pursued.