Automatic Simultaneous Multielement Analysis of Microvolume Samples with an Inductively Coupled Plasma Source

CONSTANCE BUTLER SOBEL 1800 Altadena Drive, Pasadena, California 91107

Index Headings: Microliter sampling; Spectroscopic technique; ICP technique.

Many laboratories are in need of a technique to au- tomatically analyze milligram quantities of samples for trace elements. The recent cutbacks in resources for clinical and forensic laboratories have produced prob- lems in the number of microvolume samples that can actually be analyzed per day. The analytical methods used in these laboratories are for single element deter- minations and are often operator-dependent. These methods are both costly and time-consuming. Further- more, the government has restrictions in some labora- tories for the handling of and exposure to certain types of samples, e.g., radioactive materials. An automated technique for the simultaneous determination of trace elements in microvolume samples would greatly reduce the time and cost of analysis. It would also diminish considerably the risks in the handling of and exposure to some samples.

The best technique for simultaneous analysis of trace elements in microvolume samples uses an inductively coupled plasma (ICP) source with a direct reading spec- trometer. This source provides the sensitivity needed, and the direct reading spectrometer allows multiele- ment analysis. Several investigators have used the ICP to analyze microvolumes of various materials. 1-s Most of the studies, however, were not devised for automatic sample introduction. The flow injection technique used by Greenfield s does provide for automatic sample intro- duction but the equipment is very costly.

The analysis of microvolume samples has two strin- gent requirements for sample introduction into an ICP. First, the sample should enter the plasma without being contaminated or diluted. Second, the time the sample enters the plasma should be reproducible. The emission from the plasma is to be measured only during the time period the sample is in the observation zone of the spec- trometer. The residence time of the sample in the plas- ma is only a few seconds, depending on the size of the sample.

This paper will address the method of meeting these requirements and will present data for the automatic microvolume analysis of several types of materials.

An ICP instrument (Bausch & Lomb), with simulta- neous multielement determination capability and an au- tosampler, is used for this procedure. The autosampler is adapted so that the computer on the 34000 controls the automation. The sample holder moves into position

Received 2 May 1983.

AIR SLUG

SAMPLE - -

AIR S L U G -

RINSE - - ~ /

TO PUMP ~ - . . . .

SAMPLE CUP

- --- RINSE

DIP TUBE

RINSE HOLDER

FIG. 1. Diagram of microvolume sample introduction.

and the dip tube is raised and lowered into the rinse and sample alternately. The autosampler handles approxi- mately 100 samples sequentially. The standard dip tube is modified. A smaller Teflon capillary is inserted into the standard Teflon tube. Sarstedt cups (#73.644; 5 mL; 26 x 14 mm) are used to hold the microliter sample so- lutions. The sample cups provided with the autosampler are holders for the Sarstedt cups. Samples are manually pipetted into the micro-cups with Eppendorf pipettes prior to analysis. A peristaltic pump is used to control the rate at which sample enters the plasma. A strip chart recorder is connected to the instrument to monitor the transit of the sample into the plasma in order to set the integration window effectively.

The sample is transported into the plasma as a "slug"

TABLE I. Experimental facilities and operating conditions.

Instrument

Slits

Nebulizer

Spray Chamber

Gas flow rate

Observation height

Autosampler

Peristaltic Pump

Bausch & Lomb model 34000 Vacuum Quantometer and a rf Generator (27.12 MHz band) operated 1200 w

15-#m primary slit; 50- ~m exit slits

Concentric pneumatic (Meinhard type) with pumping--an uptake rate of 1.5 mL/min

All glass conical spray chamber (volume = 90 mL

Plasma coolant argon, 12 L/min; auxiliary plasma argon, 0.8 L/ min; aerosol carrier argon, 1.0 L/min; optical transfer argon, 1.0 L/min

Centered 15 mm above the load coil

Gilson model SC15/ TD15T (with 15 mL sample cups)

Gilson model Miniplus #2

444 Volume 38, Number 3, 1984 0003-7028/84/3803-044452.00/0 APPLIED SPECTROSCOPY © 1984 Society for Applied Spectroscopy

TABLE II. Comparison of detection limits.

Element

20% HC1

Wave- Continuous 100 #L length nebulization nebulization (nm) (~g/mL) (~g/mL)

B 249.68 0.041 0.036 Cr 205.05 0.003 0.017 Cu 324.75 0.002 0.003 Fe 259.94 0.005 0.011 Ni 231,6 0.006 0.018 Na 598.59 0.014 0.16 Zn 213.86 0.002 0.005

and separated from the rinse solution by air bubbles (Fig. 1). Sharp edges are needed on the sample "slug" so the time it enters the plasma is reproducible. In order to accomplish this, four criteria need to be met. First, the capillary dip tube o.d. must be small enough so that no drops adhere to the sides after it is removed from the rinse. A larger o.d. capillary tube tends to collect drops which roll down the dip tube into the opening and cause bubbles to occur at the front edge of the sample "slug". Second, a peristaltic pump is essential to control the rate at which the sample enters the plasma; otherwise the viscosity of the sample solution is the controlling factor. Third, the tubing used in the peristaltic pump must be very flexible for tight connections and to eliminate puls- ing in the plasma source. Tygon tubing is not flexible enough. The silicon tubing manufactured by Altex, Camberly, England for a peristaltic pump is used. Fourth, the transport tubing should be as continuous as possible. Multiple connections allow the possibility of air entrain- ment and bubble formations in the capillary tubing.

The transit time and entrance of the sample "slug" into the plasma is controlled by adjusting the rate of travel with the peristaltic pump. The integration win- dow is set to coincide with the appearance of the sample in the observation zone in the plasma. The voltage across a resistor from the photomultiplier anode to ground is monitored so that when the "sample" slug enters the observation zone, a voltage drop is indicated on a strip chart recorder. The recording in Fig. 2 shows the delay time before the sample is extracted (air bubble enter- ing); the extraction of the sample (8 s for a 100-#L sam-

=o

8

8 >

Integration

Sampling Pre Flush

- - J I i ,

;8 sect , 24 sec ,[•8 sec_~ TIME

Fic. 2. Diagram of recorder tracing used to set the integration win- dow.

ple); the delay after the sample (second air bubble en- tering); the preflush time, which is actually the transport time for the "slug" to pass through the tubing; and the 8-s integration window. All of the eleven signals record- ed here are within the 8-s window.

The experimental facilities and operating condi- tions used to analyze microliter samples are shown in Table I.

The automatic microliter analysis technique was com- pared to the direct nebulization technique. Detection limits were measured and several National Bureau of Standards (NBS) reference materials were analyzed for trace elements. Table II shows a comparison of detection limits. In dissolution procedures for microsamples the concentration of acid in the final solution is relatively high. Therefore, the detection limits were measured in the presence of 20% HC1. The same pumping rate was used for both continuous and microliter nebulization. The data obtained with microliter nebulization were comparable to the data generated with continuous ne- bulization.

The accuracy of the technique was determined by analyzing NBS standard reference steel samples. Con- centration curves were prepared from synthetic stan- dards. The synthetic standards were made by adding analyte elements to pure iron solutions. The total salt concentration was 0.5 %. No attempt was made to match the overall composition of the steel samples to be ana- lyzed. A typical concentration curve is shown in Fig. 3. Each point on the curve represents the intensity ob- tained for the concentration of Ni in one 100-#L sample of each standard.

Table III shows the results obtained for the automatic

TABLE III . Analysis of NBS steel SRM's

Wt % A1 Cr Cu Mn Ni

33c ML a (3 Ni) Con b

NBS

129b ML o (Bessemer-1% C) Con b

NBS

l l l b ML" (1 Mn-2 Ni) Con b

NBS

0.018 ± 0.004 0.049 _+ 0.001 0.033 ± 0.003 0.73 ± 0.02 3.17 ± 0.06 0.013 0.050 0.028 0.72 3.19 0.032 0.052 0.031 0.73 3.28

(0.030-0.034) (0.049-0.056) (0.029-0.034) (0.729-0.738) (3,26-3.29)

• " 0.016 ± 0.0008 0.016 ± 0.002 0.74 ± 0.013 0.016 ± 0.002 ' " 0.015 0.013 0.74 0.014 • '- 0.016 0.013 0.703 0.013 • " (0.014-0.019) (0.012-0.019) (0.753-0.770) (0.011-0.019)

0.031 ± 0.002 0.071 _± 0.005 0.021 ± 0.002 0.70 ± 0.005 1.77 ± 0.02 0.025 0.069 0.025 0.68 1.92 0.043 0.070 0.028 0.706 1.81

(0.041-0.047) (0.066-0.074) (0.025-0.032) (0.695-0.711) (1.81-1.83)

" ML = Microliter nebulization. b Con = Continuous nebulization.

APPLIED SPECTROSCOPY 445

I N T E N S I T Y ( m y )

3 0 0 0

2 0 0 0 "

1 0 0 0

NICKEL CALIBRATION CURVE NI 2 3 1 . 6 n m

I I I CONCENTRATION (%) 2.0 6 .0 10.

FIG. 3. Diagram of a typical concentration curve obtained.

analysis of the microliter volumes of the NBS-SRM's (Standard Reference Materials) compared to those with continuous nebulization of the same solutions. The data for the SRM's represent the average of four 100-#L sam- ples. The results show that the microliter technique compares favorably with continuous nebulization and is in good agreement with the certified values except for Cu and A1. The data for these elements that do not agree with the NBS value are below or near the lowest quan- titatively determinable amounts (five times the detec- tion limit found in a 0.5 % steel solution).

Three other types of NBS-SRM samples were also analyzed with the microliter technique. Table IV shows the average value obtained for five 100-#L samples, five 100-~L samples, and one 100-/~L sample for orchard leaves, sediment, and coal fly ash, respectively. The re- sults obtained for all elements in these materials are in good agreement except for Na in orchard leaves and Ni in sediment and coal fly ash. An Na contamination may have occurred in the dissolution procedure. No at tempt was made to make any interference corrections for any of these three samples, which may account for the high results for Ni in the River Sediment and the Coal Fly Ash samples.

Short-term precision of the method was determined by nebulizing eleven 100-#L portions of a steel sample into the plasma and measuring the concentration for four elements. The short term precision was approxi- mately 2 % for all the elements.

During a period of 57 days, data were collected on 5

TABLE V. Long term precision.

No. of Cr Mn 100-#1 average average

Date Sample Samples wt. % wt. %

12/2 111-2 4 0.071 0.68 12/3 111-1 3 0.072 0.72

111-2 3 0.072 0.72 12/15 111-1 1 0.079 0.78

111-2 1 0.076 0.75 12/18 111-1 1 0.079 0.78

111-2 2 0.070 0.69 111-3 1 0.074 0.73 111-4 1 0.070 0.71 111-5 1 0.078 0.80

1/28 111-2 11 0.068 0.71

0.074 ± 0.004 0.73 ± 0.04

days on the same sample for a long-term precision study. Table V shows the data obtained for five separate dis- solutions of the 111 b steel. These dissolutions contained different concentrations of HC1 ranging from 5 to 10 % and total salt concentrations ranging from 5000 to 10,000 /~g/mL. Standard solutions used to analyze the sample solutions contained 5 % HC1 and a total salt concentra- tion of 5000/~g/mL. This shows that by pumping the solutions, a compensation is accomplished for the dif- ferences in viscosity which may be caused by the acid concentration or total salt concentration. The long-term precision of the automatic technique for the five ele- ments determined was 5-7 %.

The stability of a microliter sample was studied after it was pipetted into the cup to determine how much, if any, evaporation occurred with time. A sample which had been allowed to stand for 2 h was analyzed and the results obtained were compared to data obtained 2 h earlier for the same sample. The concentration of A1 and Ca at 0 h and 2 h later was determined as 5.2 ± 0,17/~g/ mL vs. 5.13 #g/mL for A1 and 33.6 ± 0.8/~g/mL vs. 32.6 #g/mL for Ca. These data show that very little evapo- ration occurred in that time period. If it is necessary to pipette the samples into the cup more than 2 h ahead of time, an internal reference can be added to the sam- ple. The intensities of the elements of interest are ra- tioed to the intensity of the internal reference to correct for any concentration change that might occur upon standing. The precision of the technique can also be improved by a factor of 7 when an internal reference is used. The RSD for the intensities of Cd and Zn are 3.4 % and 3.3%, respectively. If the intensities of Cd and Zn

TABLE IV. Analysis of NBS-SRM samples

Mn Zn Cu Ni Cr Fe Na Sr B

River sediment 793 -+ 52 1768 +_ 158 108 ± 11 NBS 785 -+ 97 1720 -+ 169 109 _+ 19

Coal fly ash 388 213 124 NBS 493 _+ 7 210 +_ 20 128 +_ 5

Orchard leaves 101 _+ 5 25 -+ 1 11 -+ 1 NBS 91 _+ 4 25 -+ 3 12 -+ 1

#g/g

57.8 ± 7.7 45.8 ± 2.9

330 131 +_ 2

1.3 -+ 0.6 1.3 _+ 0.2

96 98+_ 3

2.2 -+ 0.2 2 . 6 ± 3

301 -+ 8 300 -+ 20

206 ± 21 8 2 ± 6

39_+2 37_+1

37_+3 33_+3

448 Volume 38, Number 3, 1984

TABLE VI. Comparison of glasses.

Window glass Green bottle Clear bottle

Ca/Si Qual. 0.351 0.623 0.614 Ca Conc. 3.87 % 7.43 % 7.56 %

A1/Si Qual. 0.081 0.093 0.084 A1 Conc. 1.13 % 1.61% 1.17 %

Mg/Si Qual. 0.108 0.055 0.060 Mg Conc. 2.12% 0.10% 0.24%

Fe/Si Qual. 0.077 0.099 0.075 Fe Conc. 0.091% 0.16% 0.067 %

Ti/Si Qual. 0.059 0.050 0.055 Ti Conc. 0.064 % 0.01% 0.03 %

Cr/Si Qual. 0.062 0.101 0.065 Cr Conc. 0.016 % 0.13 % 0.013 %

Sr/Si Qual. 0.085 0.074 0.081 Sr Conc. 0.011% 0.007 % 0.009 %

are ratioed to an internal reference, Cu for instance, the RSD for the ratios of Cd/Cu and Zn/Cu are .49% and .67 %, respectively. However, an internal reference is not always desirable, e.g., when high sensitivity is required. The addition of an internal reference dilutes the sample and sensitivity is lowered. Also, with the addition of an internal reference, the possibility of contamination may occur.

Qualitative analysis of microsamples for forensic in-

vestigations can be performed when an internal refer- ence is used. Glass is a material often analyzed for fo- rensic reasons. Small chips (approximately 2 mg) of glass from three different glass items were dissolved with the H3POt acid dissolution technique2 The net intensities of the elements detected were ratioed to the net inten- sity obtained for Si. The differences in composition can be determined effectively by comparing intensity ratios. Table VI shows the data obtained for the three glasses. Included also are the data obtained when the glasses were analyzed quantitatively. The qualitative data agree with the quantitative data and the glasses can be easily distinguished from each other.

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APPLIED SPECTROSCOPY 447