Instant Notes: Analytical Chemistry

Most analytical instruments generate analog detector signals in the form of a

varying voltage or current. To store and process the signal, it must first be digi-

tized at an interface between the instrument and the computer using an analog-

to-digital converter(ADC). The detector signal may vary relatively slowly with

time, as with an autotitrator or a UV/visible spectrometer, or very rapidly, as

with a capillary gas chromatograph linked to a mass spectrometer. An ADC

must be capable of sampling and converting the sampled signal to a digital

value at an appropriate rate (in as little as a few microseconds) so as to be ready

for the next sample, and the digitized record should be as accurate a version of

the original analog signal as possible. This normally requires an ADC with

between 10 and 16 bit resolution. For example, an analog signal that varies

between 0 and 1 V and is digitized with a 12-bit ADC would produce corre-

sponding digital values in the range 0 to 4095, giving a resolution of 1 in 4096

(2^12 =4096), or voltage increments of 2.44 × 10 −^4 (∼0.2%).

If the computer is to control instrumental operating parameters, or if an

analog data output is required, the reverse process of digital-to-analog conver-

sion(DAC) is used. As for ADC, a DAC should have a resolution of 10 to 16 bits

to ensure the generation of an acceptable analog signal.

Electronic connections beween analytical instruments, databases, storage media

and other devices to facilitate the transfer, archiving and retrieval of results and

other information are known as networks. Customized software enables reports

to be generated in a desired format, and library databases to be accessed to

assist in the interpretation of results. Samples passing through the laboratory

can be logged, and other information concerned with its organization, such as

statistical assessments of the workload, and monitoring the performances of

individual instruments can also be controlled by a Laboratory Information and

Management System (LIMS) software package. Networks may be localized in

one or a small group of laboratories (local area networks, LANs) or can extend

throughout a national or international organization by use of the internet or

Compuserve.Figure1 is a diagram of a typical LAN.

Networking and
laboratory
management

332 Section H – Sensors, automation and computing

LIMS

Spectral

database Photometer

Sample

preparation

Atomic

Robot absorption HPLC

Fig. 1. Diagrammatic representation of a networked laboratory (LAN).