version 1.2 september 2012 · qui-1025 isdp manual 13 5. isdp operation for dry purging only if the...

QUI-1025 ISDP Manual

ULTRA Internal Standard / Dry Purge Manual

Version 1.2

September 2012

QUI-1025 ISDP Manual

Contents

1.1.1.1. IntroductionIntroductionIntroductionIntroduction ........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................ 1111

2.2.2.2. Gas connectionsGas connectionsGas connectionsGas connections............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................ 2222

2.1. Internal standard calibration gas..........................................................................................................2

2.2. Supply of purge (carrier) gas to the ISDP..............................................................................................3

3.3.3.3. Software setupSoftware setupSoftware setupSoftware setup .................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... 4444

4.4.4.4. Operation of the ISDPOperation of the ISDPOperation of the ISDPOperation of the ISDP............................................................................................................................................................................................................................................................................................................................................................................................................................................................ 6666

5.5.5.5. ISDP operISDP operISDP operISDP operation for Dry Purging onlyation for Dry Purging onlyation for Dry Purging onlyation for Dry Purging only ............................................................................................................................................................................................................................................................................................................................................................13131313

QUI-1025 ISDP Manual 1

1. Introduction

The internal standard / dry purge (ISDP) accessory for the Markes UltrA thermal desorption autosampler is

used to load a gas phase internal standard onto sorbent tubes loaded into the autosampler flowpath. It is also

used to dry purge humid samples. Standard and/or purge gases are introduced onto the tube from the

sampling end - i.e. gas flows onto the tube in the same direction as that used to collect sample. The tube and

flow path are leak tested before purge and/or standard gases are introduced. If a leak is detected the tube is

immediately returned intact to the tube tray (without purging or standard addition).

Following internal standard introduction / dry purge, the process of 2-stage thermal desorption proceeds

normally with pre-purge, tube desorption, etc.


2. Gas connections

2.1. Internal standard calibration gas

The ISDP accessory requires a pressurised cylinder containing an appropriate calibration gas. The cylinder

must be equipped with appropriate (inert) step-down gas pressure regulation fitted with a length of 1/-8-inch

or 1/16-inch clean, stainless steel connection tubing. (Note that the length and volume of pressure regulation

equipment and connecting tubing should be kept as small as possible.) The pressure of internal standard gas

required to be delivered to UltrA is typically in the range (10-20) psig, and should be set (just (~2 psig) below

the pressure of the purge gas - see below).

A suitable internal standard would contain one or more gas-phase organic components that behave in a

similar way to the target analytes of interest, but are not found in the sample. Deuterated toluene is commonly

selected as an internal standard for BTX analysis on single-bed sorbent tubes. A mixture of deuterated n-

butane and deuterated n-octane could be used for calibration of a tube packed with two sorbents - e.g.

Carbograph 1 and Carboxen 1000 - used to monitor volatiles from n-C3 to n-C12.

The volume of calibration gas introduced onto the tube is determined by a sampling loop on a gas sample

valve incorporated into the ISDP. The standard loop supplied with the ISDP is 1 ml volume. This means that

the concentration of internal standard gas in the pressurised cylinder should be such that a 1 ml volume of

internal standard gas introduces a similar mass of internal standard compound as the mass of target analytes

collected during a typical monitoring exercise. For example, if the ISDP is to be used for calibration of 10 L

volumes of real air containing compounds at ~1 ppb concentration, the concentration of internal standard

should be approximately 10 ppm. This is because the mass of component in 1 ml of internal standard gas at

10 ppm is approximately equal to the mass of analyte(s) that would be collected from 10 L of air at

approximately 1 ppb.

The standard gas cylinder is connected to an 1/8-inch,

stainless steel bulkhead fitting located on the rear

panel of the autosampler labelled A.

Note that if 1/16-inch narrow bore connecting tubing is

being used appropriate reducing unions will be

required.

As soon as the cylinder has been connected to the

bulkhead on ULTRA it may be opened. An on/off

solenoid valve located downstream of the sampling

loop prevents flow out of the cylinder with the ULTRA–

UNITY 2 in standby mode.

The flow of calibration gas through the loop is

controlled using a needle valve B and and measured at

vent C.

Pneumatic gas is connected via 4 mm plastic tubing to

the autosampler at 50 psi.

A

B

C


2.2. Supply of purge (carrier) gas to the ISDP

The purge gas is used to sweep the contents of the loop onto the sorbent tube in the sampling direction and

also to purge the entire ISDP flow path, keeping it free of contamination. The pressure of the purge (carrier)

gas is regulated using conventional pressure pneumatics (and is usually set just above (~2 psig higher than)

the pressure of the calibration gas and just below (~2 psig lower than) the pressure of the carrier gas

supplying the UNITY thermal desorber and GC-column.) Purge gas pressure is typically set in the range 12-22

psig.

An independently regulated supply of pure purge gas -

usually carrier gas - is also required to be connected to

the appropriate 1/8-inch brass bulkhead on the rear

panel of the autosampler D.

The purge gas flow can be measured and adjusted

using the appropriate vent F and needle valve E shown.

D

E

F


3. Software setup

To access the various control parameters, the ISDP accessory must be enabled in the software.

Go to View > Options

Go to the Configuration tab select and select

‘Enable’ in the Internal Standard section.

Ensure the tube ‘Tube Load Temperature’ is

low enough to prevent a loaded tube heating

up significantly during the leak test, prior to

internal standard addition or dry purging.

A typical value for this temperature is 50 °C.


In Standard 2(3) stage desorption methods

several options are now available.

1. Dry Purge

A flow of carrier gas for dry purging sampled

tubes with a time in minutes

2. Inject standard

Upon selection of Inject Standard two

parameters become available; the inject

standard time and the loop fill time.

The Inject Standard time is the time, in

minutes, for which the internal standard loop is

purged with carrier gas to transfer the

standard to the sample tube.

The loop fill time is the amount of time, in

minutes, that the internal standard gas is

purged through the standard loop prior to

loading onto a sample tube. The value should

be long enough to ensure that, given the flow

of calibration gas set using the needle valve,

the regulators, connecting tubing and gas loop

are completely purged with calibration gas.

Enter the other desorption method parameters

for the method as described in the main UNITY

2 Operators manual.


4. Operation of the ISDP

The ISDP accessory is controlled within the main

software. The process of internal standard addition

involves the stages illustrated schematically below.


Whether the ISDP is being used for dry purge only or

internal standard addition, the tube loaded into

ULTRA is first leak tested to ensure the integrity of

the tube seals and all other parts of the flow path.


If the tube passes the leak test and if standard

addition is required, the software then actuates a

solenoid valve to sweep calibration gas from the

cylinder and into the loop. Exhaust gases from this

part of the process are purged to vent. As described

above, It is important to allow sufficient loop fill time

to ensure that the composition of gas standard in

the loop at the end of loop fill time is representative

of that in the cylinder.

Given that the total void volume of the cylinder

pressure regulators, connection tubing to ULTRA and

the gas loop itself may be 10 ml or more, and given

also that ten times the volume of gas is needed to

completely purge one gas with another, calibration

gas flows and loop fill times are typically set at 50

ml/min and 2 minutes respectively.

NoteNoteNoteNote: After initial setup this is the stage where the

internal standard flow can be adjusted – it cannot

be adjusted while in standby.


At the end of the loop fill time (if applicable) the gas

sampling valve moves and the supply of purge

(carrier) gas is opened to sweep (inject) the contents

of the loop onto the tube from the sampling end or

to begin the dry purge phase. The purge gas flow

used to load the standard onto the sorbent tube is

set by manually adjusting the purge flow needle

valve on the ULTRA bulkhead, see Section 2.2.


At the end of the 'Inject Standard’ time / ‘Dry Purge’

time, the sequence continues with the next step of

conventional 2-stage thermal desorption - i.e. pre-

purge.

Note that the normal purpose of the pre-purge stage

is to purge air from the adsorbent prior to

desorption. However, given that purge (carrier) gas

has been purged through the tube during 'Std

Injection' / 'Dry Purge', the pre-purge time may be

minimised to 0.1 minutes to reduce the TD cycle

time if required.


Following pre-purge, the sample tube is desorbed in

the normal way.


Just prior to the end of tube desorption, the short

length of narrow-bore connecting tubing used to

deliver calibration gas during the 'Inject Standard’

phase is purged with carrier gas. This is stage is

called 'Line Purge'. It is reported in the status bar of

the operating software as 'ULTRA cleanup'. The

primary tube desorption time can be extended to

account for the line purge time if required, but this is

not usually necessary.

The final thermal desorption steps are identical to

normal ULTRA – UNITY 2 operation.

After desorption the tube is unloaded at the

temperature specified and returned to the tray as

normal - See main ULTRA Operators manual.


5. ISDP operation for Dry Purging only

If the ISDP is only to be used for dry purging tubes, no cylinder of gas-phase internal standard is required.

However, an independently regulated supply of pure purge (carrier gas) must still be connected to the

appropriate 1/8-inch brass bulkhead on the back of UltrA.

In this mode, there is no equivalent time to the loop fill time used during internal standard addition, but pure

purge (carrier) gas does flow through the ISDP accessory and onto the sampling end of the sorbent tube

loaded into the autosampler, in a stage called 'Dry Purge' which is analogous to the 'Inject Standard’ time

described above.

As above, the flow of purge gas is measured and controlled by manually adjusting the purge flow needle valve

on the bulkhead at the back of UltrA and the 'Dry Purge’ time is set within the software.

Note that dry purging of a sample tube loaded into UltrA will not take place until that tube has successfully

passed the leak test.

After the dry purge stage has been completed, UltrA –UNITY 2 will resume the normal steps of 2-stage thermal

desorption operation. Note that if the ISDP is being used for dry purging tubes, the flowpath through the ISDP

accessory is purged briefly with purge (carrier) gas towards the end of tube desorption to ensure all parts of

the flow path are kept clean and free of contaminant ingress.

As the operation of the ISDP accessory is determined by entry of the 'Dry Purge', 'Inject Standard’ time or 'Std

Inject Inactive' parameters as described above, it is possible to generate an automatic thermal desorption

sequence containing multiple methods - some of which include standard addition, some dry purge and some

with neither.

Note that selection of a long (e.g. 10 minute) 'Inject Standard’ time effectively also dry purges tubes that are

being loaded with internal standard. The minimum volume of purge gas required to adequately transfer

calibration standard from the gas loop to the sample tube is at least 50 ml and 'Inject Standard’ and flows of

calibration gas, must be set accordingly.

version 1.2 september 2012 · qui-1025 isdp manual 13 5. isdp operation for dry purging only if the...

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